Chip select is the name of a fairly common input signal for the chips that make up various computer peripherals such as memory, I/O devices, etc.
The chip select signal is a mechanism by which a bus controller or address decoder can notify a specific peripheral when its address is being talked to by the processor. This allows for one model of chip to work with any number of different processors with a minimum of glue logic.
By convention chip select lines are usualy active low, and are represented by this symbol:
The bar over the CS means "active low". Some times you'll see it as: