An audio limiter is basically a compressor on steroids. Once the compression ratio goes above 10:1 on a compressor you have limiting. As a reminder a compressor turns down the peaks of an audio signal, which allows the overall gain to be increased making the audio sound less dynamic (or less extreme in loud and soft volumes). It also increases the average volume of the audio making the audio be perceived as sounding louder to the human ear. Limiting simply makes the results more extreme by not allowing much signal (if any) to go over the threshold. As mentioned earlier, once you set a compressor above a 10:1 ratio it is limiting. This can be called soft limiting while pushing the ratio to 100:1 or even infinity:1 becomes hard limiting or brick wall limiting. This kind of limiting is often used in mastering to get tracks super loud.