In Sonnet 90, Petrarch expresses two main ideas about love: one is that his love for Laura does not fade over time and as she ages; the second is that his love for her is painful because it is not returned. 

Petrarch's first idea -- that Laura remains beautiful and desirable to him over the years -- is conveyed mostly through physical descriptions of Laura, using imagery and figurative language. Stanza one (lines 1-4) provide a good example of imagery: 

Upon the breeze she spread her golden hair
that in a thousand gentle knots was turned
and the sweet light beyond all radiance burned
in eyes where now that radiance is rare

Here, Petrarch describes Laura's hair blowing in the wind in a way that allows readers to picture the image. This excerpt also ends with a contrast -- Petrarch notes that Laura's eyes no longer show the same "radiance" they did when she was younger. Petrarch continues to discuss his physical attraction to Laura in lines 9-13: 

She moved not like a mortal, but as though
she bore an angel's form, her words had then
a sound that simple human voices lack;

a heavenly spirit, a living sun
was what I saw

Here, Petrarch uses figurative language to compare Laura to an angel and place her on a pedestal above other women. He equates Laura to "a heavenly spirit, a living sun," and these metaphors indicate how highly Petrarch thinks of her.

However, Laura never returned Petrarch's love, and the pain he feels as a result can be seen in the  middle and at the end of the sonnet. In lines 5-8, Petrarch writes: 

and in her face there seemed to come an air
of pity, true or false, that I discerned:
I had love's tinder in my breast unburned,
was it a wonder if it kindled there?

In this stanza, Petrarch perceives in Laura's face "an air/ of pity," so even though he is not sure what she feels, he interprets Laura's look to mean that she feels sorry for him (likely because she will never love him and he so obviously admires her). Petrarch uses a metaphor in lines 7-8 to describe the passion he feels for Laura: he has the "tinder" to begin the fire of love, and it is "kindled" because Petrarch feels attracted to Laura. However, the tinder is "unburned" because it is not shared by Laura. Finally, at the end of the sonnet, Petrarch writes,

now, if it is not so,
the wound's not healed because the bow goes.

He feels injured or damaged because Laura does not return his love. The "wound" can never be healed, unless she were to love him. In Sonnet 90, Petrarch uses imagery and figurative language to both express his continuing love for Laura and the pain he feels because that love is unrequited. 

Petrarch's Sonnet 90 says, in subtext, that true love must be sought and that action must be taken if one is to have a chance of experiencing true love. As noted above by Karen P.L. Hardison, the sonnet speaks of this man who did not take advantage of the opportunity for true love presented to him in his life. This man missed his chance with her and "the wound's not healed."

Petrarch's Sonnet 90 does present the feelings of true love, even though this man has not really experienced it. His reminiscences of the woman are manifestations of a true love locked inside him. He didn’t take action; he is now left to ponder the beauty of the woman, her essence. This is evident in the line:

She moved not like a mortal, but as though

she bore an angel's form...

The man also speaks of the woman’s voice. It was a voice beautiful and exclusive, unlike anyone else’s. This is how one typically thinks when thinking of a true love – that their qualities, characteristics, and personality are truly one-of-a-kind. This is what draws one individual to another – this recognition and acceptance of their uniqueness.

Poetic techniques used in Petrarch's Sonnet 90 include the sonnet form itself. The fourteen-line rhyme scheme is ABBA, ABBA in the first two stanzas (4 lines each). For the last two stanzas (three lines each) the rhyme scheme is CDE FCC, a variation on the usual CDE CDE or CDC DCD rhyme scheme.

This sonnet also employs alliteration:

…radiance is rare

… a sound that simple

… was it a wonder

In addition, the technique of metaphor is used in this sonnet. The object of the man’s desire is being identified like or compared to “a heavenly spirit, a living sun” and this reveals what the man truly sees her as. It also reveals that he realizes he has let her out of his life to his chagrin.

Petrarch's Sonnet doesn't speak of true love, it speaks of unrequited love that lives on in painful memories. True love is most logically equated with love is reciprocated between two people. In this Sonnet, The speaker reminisces about a bygone time when he watched the object of his affection, to whom he had not declared his affection ("I had love's tinder in my breast unburned, / was it a wonder if it kindled there?" or according to Norton's, "I had love's tinder heaped within my breast; / What wonder that the flame burnt furiously?") and he describes her and what he remembers seeing. His descriptions are examples of the literary technique (one of two literary device categories) of hyperbole in which things are exaggerated. For instance, "a thousand gentle knots" and "sweet light beyond all radiance burned / in eyes... " are both examples of the exaggeration of hyperbole.