What's he saying?
"My tongue-tied Muse in manners holds her still, / While comments of your praise richly compiled,"
My muse does not inspire me, though treatises praising you are put together with great learning,
"Reserve thy character with golden quill, / And precious phrase by all the Muses filed."
Prasing you in writing that is polished and smoothed by all the nine muses.
"I think good thoughts, whilst others write good words, / And like unlettered clerk still cry 'Amen'"
I merely think about you while rival poets write about you, and like a low-ranking illiterate, I agree with them
"To every hymn that able spirit affords, / In polished form of well-refined pen."
I support every paean of praise offered by a gifted and talented person.
"Hearing you praised, I say ''tis so, 'tis true,' / And to the most of praise add something more;"
[I agree when I hear people praising you, and I praise you even more than that;]
"But that is in my thought, whose love to you, / Though words come hindmost, holds his rank before."
[But that which I add is in my mind, and those thoughts rank above all words.]
"Then others, for the breath of words respect, / Me for my dumb thoughts, speaking in effect."
So respect the rival poets for their meaningless words, and respect me for my valuable thoughts, though they do not speak.
Why is he saying it?
Sonnet 85 is one of the "rival poet" sonnets, in which the speaker is preoccupied with the other poets who seek to praise the fair lord. It is ironic in that it seems to undermine itself simply by existing: the speaker asserts that the other poets "write good words," while he adds more value to his praise of the fair lord by his thoughts; however, Sonnet 85 is written down, thus disproving the idea it puts forth. This lends a sarcastic tone to the poem, since what the speaker seems to really be saying is that the empty "breath of words" of the rival poets do not compare to his own words.
In classical antiquity, the muses were nine goddesses, each overseeing one of the branches of poetry. If the speaker's muse is "tongue-tied," it means he is not inspired to speak in an artful way. The "precious phrase by all the Muses filed" refers to any poetic offering by the rival poets. The term "golden quill" in line 3 adds to the idea that the praise of the rival poets' work is a hyperbole. If their words are "filed," or smoothed over and polished by the Muses themselves, it is likely they are too good to be true, and even dishonest.
The phrase "reserve thy character" in line 3 hearkens back to the theme of preserving the fair lord by immortalizing him in poetry. The word "character" is a pun: it can mean appearance, as it does in Twelfth Night when Viola says to the Captain, "I will believe thou hast a mind that suits / With this thy fair and outward character" (I.ii.50-1). However, "character" can also mean a symbol of writing, like the letters of the alphabet, as it does in Sonnet 59: "Show me your image in some antique book, / Since mind at first in character was done."
Biblical language appears in this sonnet to praise the fair lord, as is common in other sonnets. In line 6, the "unlettered clerk" refers to a low-ranking cleric who cannot read or write. Only educated people, necessarily of the upper classes, were allowed to become clergymen, but there were not enough of them to fill all the positions; so the gaps would have been filled by uneducated laymen who knew the responses to prayers by heart; here, the poet likens himself to one of them, crying "Amen," which is the closing to a prayer. Line 7 refers to a "hymn," or song of praise.
The final four lines of the sonnet assert that the poet's thought "holds his rank before" the words of rival poets, meaning it is of higher value to the fair lord. The phrase "breath of words" seems to imply an emptiness in the praise of the other poets. This contrasts with the idea put forth in the final couplet Sonnet 81: "You still shall live - such virtue hath my pen - / Where breath most breathes, even in the mouths of men." Where Sonnet 81 suggests that the speaker writes in order to preserve the fair lord's memory "in the mouths of men," or to make sure the fair lord is always spoken about, Sonnet 85 undermines the value of words of praise.