What does "Wednesday's child is full of woe" mean?

The line "Wednesday's child is full of woe" is a part of a nursery rhyme known as "Monday's Child," sometimes attributed to Mother Goose; it predicts that children born on Wednesday are sad.

The rhyme is a fortune-telling song, predicting that children born on different days are to lead different lives. For example, children born on a Monday are said to be "fair of face," while those born on a Tuesday are "full of grace," according to the rhyme. Over the years, the fortunes associated with each day have changed. In some earlier versions, it was Friday's child that would be "full of woe." The rhyme dates back to around the 16th century.