FAQ: What is latency?

Latency
Latency is the time delay between input and output. The lower the latency the better it is for recording, however it is harder on the system and is not always possible.
Latency can make recording very difficult, wherever possible try to minimize it.
Firewire interfaces should be capable of near-zero latency (64 samples) and most USB interfaces have a zero latency monitoring control which sends the unprocessed input directly to the output. This is also called direct monitoring, I strongly advise against using this feature as you don't hear if there is a problem with the recording until it is too late. To adjust the latency of the interface you want to look for the ‘buffer size’ control. A higher buffer size is useful when mixing with a lot of plugins, and a low buffer size is preferable when recording, adjust as necessary.