The mnemonics "All Cows Eat Grass" and "Good Boys Do Fine Always" are used to remember the notes of the bass clef. They correspond to the ACEG space notes and the GBDFA line notes, respectively.
Other mnemonics include "All Cars Eat Gas" and "All Clintons Eat Goobers" for ACEG notes, and "Great Big Dogs Fight Animals" for GBDFA notes. However, these are not as popular.
The mnemonics are applied to the staff by counting either the spaces or the lines from the bottom. Mnemonics are very helpful for beginner music students, but they are usually not needed for advanced students with an understanding of music theory.