A limiter pedal is a handy tool aimed primarily at bass guitarists that allows you to play without the fear of obtrusive loud notes cutting through, potentially unbalancing your performance and distorting. By capping the maximum volume level, a limiter creates a “brick wall” against which your signal comes up against, keeping any unruly loud moments in check.
Whereas a compressor pedal takes your signal, squashes its dynamics and then boosts its volume, a limiter allows you to push your signal up until it meets the limit wall you set. Any dynamic peaks that meet this wall are denied advancement and will not get any louder, while lower dynamic frequencies can continue to advance towards the wall and increase in volume. This allows your dynamic range to remain intact, only affecting the highest peaks.
Basically, it stops your loud bits from being too loud. Buy one, you’ll not regret it.