L stands for Latched, while U stands for Unlatched.
The FS-5L is a latched footswitch that will click ON when you step on it and release, and OFF when you step on it and release a second time. This switch is designed for applications like channel switching on a guitar amp.
The FS-5U is an unlatched switch that will stay ON only as long as you are depressing the pedal. This type of footswitch is designed for uses such as a sustain pedal.