FOH = Front of House
This acronym is frequently used to describe sound systems used to deliver amplified sound to the audience area. This distinguishes it from sound systems used to deliver amplified sound to the stage (stage monitors, or simply "monitors").
The L1® approach, with the L1®s on the stage behind the performers allows the L1®s to act as both Front of House system and stage monitors.
More general information about FOH / Front of House in wikipedia