What Does This Mean?

I think he is talking about his lover getting older. He says that everything taken from her by Time (i.e., the aging process), he will give back to her. Engraft means 'graft', as to graft a tree of a rose bush. They get a new tree to grow on the old root or stump, by binding them together. Many fruit trees are done like this, to combine the hardiness of one variety with the fruit of another. So Shakespeare is using this as a metaphor to say he can give her a new young body on the old 'stump' as it were. You could extend that to a ual meaning if you took it as he would give her a baby, but I don't think that is what he intends, really.