What's he saying?
"What's in the brain, that ink may character, / Which hath not figured to thee my true spirit?"
What is left to write about, when I have already written so much to show you my love and emotions?
"What's new to speak, what now to register, / That may express my love, or thy dear merit?
What else is there to say to express my love for you, or to praise how wonderful you are?
"Nothing, sweet boy; but yet, like prayers divine, / I must each day say o'er the very same;"
There's nothing new to say, but like we do with prayers, I repeat the same words over and over again;
"Counting no old thing old, thou mine, I thine, / Even as when first I hallowed thy fair name."
Our relationship still feels as new as when I first wrote about you.
"So that eternal love in love's fresh case, / Weighs not the dust and injury of age,"
Love does not take into account the effects and failings of old age,
"Nor gives to necessary wrinkles place, / But makes antiquity for aye his page;"
Neither does it give importance to inevitable wrinkles, but rather the loved one sees the example of how ancient records have always praised what is beautiful;
"Finding the first conceit of love there bred, / Where time and outward form would show it dead."
Since those records reflect love as young, though obviously the records themselves are old.
Why is he saying it?
Sonnet 108 is significant because there were 108 sonnets in Sidney's Astrophel and Stella, one of the earliest and most influential sonnet sequences published in England. Published posthumously in 1591, it had been circulating for eleven years before that. Many of Shakespeare's sonnets echo ideas or specific sonnets in Astrophel and Stella, and so it makes sense that the 108th sonnet would be the point at which the poet asks, "What more is there to say?" Rather than ending here, however, he concludes that since love is eternal and will always remain new and fresh, the same words of devotion can be repeated without losing meaning.
This sonnet can be grouped with Sonnets 105 and 106, in that the poet uses them to discuss the importance of things he can say about the fair lord. These sonnets contain the only three instances of the word "express" in the sonnet sequence. Sonnet 105 explains that the poet's focus on the beloved's being "fair, kind, and true" suffices, since his poems "one thing expressing, leaves out difference." Sonnet 106 declares that the "antique pens" of poets of long ago "would have express'd" the fair lord's beauty adequately if they could have seen it for themselves. The concentration of the usage of this word draws attention to the idea of these sonnets: that is, the extent to which love depends on being expressed in poetry.
Lines 5-8 have been called blasphemous, since they clearly compare devotion to the fair lord to devotion to God. The most obvious reference is line 8: "Even as when first I hallowed thy fair name," which is The Lord's Prayer, or the "Our Father:" "Our Father, who art in heaven, hallowed be thy name." This is the most famous Christian prayer, and the reference would not have been missed by any of the sonnet's readers. The question thus remains how Shakespeare got away with what would have been considered obvious blasphemy in such a strict society.
The "prayers divine" of line 5 seem to refer to a rosary, a repetitive prayer formula. A rosary consists of one "Our Father," ten "Hail Marys," and on "Glory Be," repeated five times. In the following line, the speaker complains that, "I must each day say o'er the very same," as one would when praying the rosary. This implication is enforced by the word "counting" in line 7. So even though on the service, these lines refer to the the poet's dedication to writing about his beloved without variation, the metaphor of the rosary is apparent.
The meaning of lines 12-14 is a bit unclear, since "page" could mean either the page of a book or a servant. The interpretation here assumes it means the page of a book, taking into account the reference to writing in line 1, as well as the "chronicle of wasted time" referred to in Sonnet 106. Thus "there" in line 13 refers to the old chronicle, and "time and outward form" means the chronicle's appearance. However, if "page" is taken to mean servant, then the meaning is that love conquers "antiquity," seeming to make Time its servant. In this case, the final couplet refers to the love between the poet and the fair lord, which still seems as young as when it was "bred," or first conceived.