William Shakespeare's Sonnet 73 is a poem about love, like many of the sonnets. In this sonnet, Shakespeare is discussing the season, autumn, and how he has grown old.
In order to interpret the lines, one must understand the imagery to which he is speaking of. The lines in question are as follows:
Upon those boughs which shake against the cold,
Bare ruin'd choirs, where late the sweet birds sang.
The boughs which Shakespeare is speaking of is referring to tree limbs. Winter weather is moving in and the wind is shaking the branches. This could be looked at as personification given the branches are shaking against the cold (much like a person would do if under-dressed for the bitterness of the weather). Therefore, line three is simply depicting branches moving about given the wind is forcing them to move.
Line four is still speaking about the branches. Here, the reader can see that the leaves have certainly fallen given the branches are described as being bare. The image Shakespeare sets up here is that the branches served as a choir stand which holds those who would sing. In regards to the birds, the birds are the choir who used to sit upon the branches and sing. Given the leaves have dropped and the weather has changed, the birds no longer perch on the branches to sing. (This is denoted by the use of the word late, meaning no longer.)