The implied listener is addressed in the first line using the direct address of "thou." As the speaker narrates the poem which metaphorically describes how he is growing older, the reader is reminded that these metaphors are intended for this ultimate person to whom the speaker addresses his thoughts. He realizes that he is in the autumn of his life, the externally beautiful period that precedes a final season of cold decay. He compares this point in his life to a sunset, breathtaking in its own right, but which exists as a harbinger of the darkest night.
The introduction of this listener is important because we are reminded that this poem was not written for a generic audience. Instead, the speaker's emotions about death and dying are intended for a specific person, someone whom the speaker trusts and shares a special bond with.
In a Shakespearean sonnet, there is typically a problem or theme which is described in the first twelve lines, and there is some type of resolution in the final two lines. In this sonnet, the speaker is dying, which is presented through various metaphors. However, in the final couplet, this issue is addressed by speaking again to this implied listener:
This thou perceiv'st, which makes thy love more strong,
To love that well which thou must leave ere long.
The speaker understands that his beloved will love him more in these final days because she realizes that their love has limited time remaining. She will thus love him "well" through the autumn and sunset of his final days, her love made "more strong" by witnessing the death of this "fire" she has found in the speaker.