Please explain specifically the kind of love that Shakespeare was talking about in Sonnet 73.
The speaker of Shakespeare's sonnet 73 seems to be the same speaker as the surrounding sonnets, and the addressee, too, is generally supposed to be the same. The sequence of sonnets to 1-126 constitute what scholars call the "Fair Youth" sequence. Sonnet 73 is among the most famous. The speaker spends 12 of the sonnet's characteristic 14 lines laming his old age ("In me thou see'st the glowing of such fire / That on the ashes of his youth doth lie"). The final two lines overtly address the love object. Shakespeare writes:
This thou perceivest, which makes thy love more strong,
To love that well which thou must leave ere long.
The neatest understanding of these is that Shakespeare was writing to a male, owing to gender designation in other sonnets. The possibility therefore exists that Shakespeare was writing to a homosexual love interest who is younger than the speaker. While by modern standards this option is unlikely, such was wholly unheard of in Shakespeare's era. Modern and pre-modern England was steeped in the classics, which featured forms of "pederasty" (love between a more mature, older man and a fair, youthful man).
Within this sonnet, it's only possible to tell that the speaker appreciates the addressee's love for him. The best analog for understanding his addressee as a man is Sonnet 20, which reads:
A woman’s face with Nature’s own hand painted
Hast thou, the master-mistress of my passion.
Thus, here and elsewhere, while it's impossible to know for sure, it seems that Shakespeare is addressing a young man, whom he loves romantically.
Most likely, he is talking about romantic love but it could also be addressed to a beloved friend. The speaker tells his beloved to envision him as autumn, then twilight and sunset. Since it is ambiguous as to whether this is a romantic love or one born out of friendship, the main type of love being described is one of urgency.
The speaker compares himself to things analogous to death: autumn and lateness of day. The speaker may be implying that he is old and this fact should make their love stronger because he won’t be around forever. In general, the sonnet is about how fleeting life is. The older you get, the more fleeting it becomes. This is true in terms of lifespan but also in terms of perspective. When you’re five years old, a year is 20% of your life. When you are fifty years old, a year is 2%. As you age, a year seems relatively less. The speaker is calling attention to this fact in order to increase the intensity of their love or friendship with what time they have left.