Addresses starting with "3", P2SH stands for Pay To Script Hash, which supports more complex functionality than Legacy addresses.
Nested P2SH, takes an existing P2SH address (starting with "3") and wraps it with a SegWit address. This is used to make it compatible with older versions. However, this is some kind of hack that causes SegWit transactions to require about 10% extra space.
For example: 3KF9nXowQ4asSGxRRzeiTpDjMuwM2nypAN