The word "this" in line 14 of Shakespeare's Sonnet 18 refers to the poem itself. Sonnet 18 is one of several sonnets in which the speaker proclaims that his love for his friend and his friend's beauty shall be eternalized in the speaker's verse. In Sonnet 17, the speaker asserts that future generations may not believe him when he describes his friend's beauty, so he urges his friend to have a child so that his beauty "should live twice, in it [the child] and in my [the speaker's] rime" (14). We see another example in Sonnet 19 in which the speaker is addressing Time, imploring it not to take his friend's youth and beauty; however, knowing that to be impossible, the speaker concludes with this statement: "Yet do thy worst, old Time: despite thy wrong / My love shall in my verse ever live young" (13-14). Finally, we see in Sonnet 65 this same theme repeated again.
Since brass, nor stone, nor earth, nor boundless sea,
But sad mortality o'ersways their power,
How with this rage shall beauty hold a plea,
Whose action is no stronger than a flower?
O, how shall summer's honey breath hold out
Against the wrackful siege of battering days,
When rocks impregnable are not so stout,
Nor gates of steel so strong, but Time decays?
O fearful meditation! where alack,
Shall Time's best jewel from Time's chest lie hid?
Or what strong hand can hold his swift foot back?
Or who his spoil of beauty can forbid?
O, none, unless this miracle have might,
That in black ink my love may still shine bright.
In all of these examples, the speaker is essentially stating that he can immortalize his friend in verse, and he is right as we are still reading these sonnets 396 years after Shakespeare's death.