Shakespeare’s speaker looks ahead to a future where his lover has aged just as he himself has now. He imagines time’s rough hand crushing and wearing him down, draining youthfulness and carving wrinkles into a once-smooth brow. That bright morning of young beauty will have travelled into the steep darkness of old age, all the treasures of spring quietly stolen away. There’s real sadness in picturing that loss, a heavy, elegiac mood that almost mourns the beloved before the fact.
But then the poem pivots. The speaker is fortifying against this future right now, building a defense so that time’s knife cannot slice the memory of that beauty away. The “black lines” — the very words we’re reading — become a living record. Though the actual lover will eventually die, his beauty will survive in these verses, kept fresh and green forever. It’s a quiet, determined refusal to let decay have the final word.