In the first four lines, Sidney admonishes the affected intellectuals who call on "the Sisters nine" (the nine muses) in order to mask their works with "flourishes" the poets themselves cannot create or, imitating Pindar, use lofty phrases and enameled, variegated (several colors) flowers to enhance the effect of their "golden thoughts."
The next four lines continue the theme of false embellishment of mundane thoughts:
Or else let them in statelier glory shine,
Ennobling new found tropes with problems old,
Or with strange similes enrich each line,
Of herbs or beasts which Inde or Afric' hold.
Again, Sidney is pointing out that these poets are using new metaphors and similes to make mundane thoughts seem original and embellishing their lines with exotic comparisons to herbs from India or beasts from Africa in order to make their poetry seem more original.
The poet then admits that he only knows one Muse and that he is restricted to simple phrases because fancy rhetoric is out of his reach and, as well, he is too poor to buy exotic things. He concludes that all he needs in order to describe beauty is to look into Stella's eyes and "copy what Nature has written in her."
In this sonnet, Sidney argues that unskilled poets have to search for costly and unusual imagery in order to describe their lady's beauty, but Sidney finds all the inspiration he needs in his lady's eyes and simply needs to describe what is there in plain words.