Pockets of Mud are pockets that contain mud.
The history of pockets of mud
One day Onion had a pair of pants and the pockets of those pants were full of mud, he decided to write a song about it called pockets of mud. It was never recorded because Onion never told Chavez about it, he intends to tell him next time they go to the studio to record their next album.
What is the song exactly about?
Nothing but mud in pockets.