An RS NOR latch is used in redstone creation to remember the input and is best used with and input like buttons and pressure plates.
An RS NOR latch is very simple when you break it down.
- One side powers the other which then disables the other side
- When power is applied to the other side it becomes powered and the original powered side turns off.
This can be used to keep a door open until you walk over another plate, in passwords and in very complex redstone creations.
An easier to understand version of the RS NOR latch can be seen below: