If the padding field at the RTP header is set to 1, it means that padding existing in the packet. The last byte of the packet denote the number of padding bytes should be ignored.
The value include itself. So, if the last byte is a value of 9, it means that the last 9 bytes of the RTP packets are padding.
Some reasons to have padding are
1. The payload are some encrypted data with certain fixed block size
2. Carrying several RTP packets in a lower layer protocol data unit
See: http://www.ietf.org/rfc/rfc3550.txt Section 5.1