Top posting is replying before the text which you are replying to (and which is usually automatically quoted by your email client).
This example from Debian wiki answers to both, this and next question.
A: Because it messes up the way you read Q: Why is top-posting bad? A: Writing your answer before the question Q: What is top-posting?
See the answer to the previous question.
It also makes harder to know what previous poster meant if he/she just said "+1" and other people have used different posting style and senders and replies are messed up. This happens often on Lubuntu mailing lists.
At least Debian mailing lists and Ubuntu mailing lists forbid top-posting in their guidelines. In my opinion other mailing lists should do this too.