Before we go into summarizing Sonnet 73, we should make one thing clear from the start: not much really happens in this poem as far as ideas are concerned. Basically, you've got one idea (the speaker is growing old, and it stinks) that runs from line 1 all the way down to line 12.
In lines 13-14, you get a different idea, though it's related to the first one. The speaker tells the person he is talking to that, because he (the speaker) is going to die soon, the other person should treasure their love all the more.
Fair enough—so why are we reading this poem exactly? Well, the thing is that, this being a poem and all, it's a lot less about what the poet says and a lot more about how he says it. In this case, how the poet says it makes all the difference. That's because Sonnet 73 is really all about the poet showing off—by using a different main metaphor in each of the three quatrains.
In quatrain 1, the main idea is all about the changing of the seasons: the speaker compares his middle-aged self to a tree that is losing its leaves in fall. In quatrain 2, he changes imagery. Now, the speaker compares himself to a fading sunset. Then, in quatrain 3, he changes things up again, this time comparing himself to the last glow of a fire in the process of burning out.
Throughout all this time, we haven't heard anything about love, or the specific relationship between the speaker and whomever he is speaking to. So when the concluding couplet comes around, it gets to have a nice surprise effect by revealing exactly the details that have remained hidden until now. This is all pretty nifty, in Shmoop's humble opinion.