Explain the poets use of the winter and summer in "What lips my lips have kissed" by Millay.

This poem is about loss, and specifically about the loss that comes inevitably as we humans age.  We cannot go back and feel the same things about the same places and the same people.  Even if we see them again, we are not the same people we were when we originally had those experiences.  So this is a poem about that kind of loss.

In poetry, of course, summer tends to represent youth and the best part of one's life while winter stands in for age and decline.  This can be seen in this poem as well.  Now that it is winter, the tree (or the speaker) can no longer know its past loves because the summer that used to sing in it no longer does.

So, now that it is winter, we are not the same people that we were in summer and we can never again be the same or feel the same as we did.  The metaphor of summer turning to winter represents that decline and loss as we age.