In many digital audio devices, such as gates, limiters and compressors, look ahead is a solution to the problem of transient waves. It allows the device to proactively change the signal - for example limiting a loud bass drum - without having to wait and react, which would allow an undesireable transient through.
A limiter is present to protect loudspeakers from overly loud signals which may break the drivers. As the sound gets louder, the limiter turns down the gain control to bring the output to a more acceptable level. A transient, however, is fast enough to get through the system before the device has had a chance to react.
A gate cuts the output when the input is below a certain level. As the input gets over the threshold - when the vocalist begins singing for example, the gate opens and the sound is allowed through. Again the reaction time is a problem, and the start of the sound may be clipped.
The obvious solution is to open the gate, or activate the limiter before the event occurs, which is the purpose of the look ahead. Unfortunately, without a time machine, this is impossible.
The practical solution is to activate the device from the raw signal, but the signal which is actually processed is delayed very slightly using a small digital delay line.
The delay is not sufficient to be noticable (it is often in the region of 200 microseconds), but is sufficient for the device to react in time to process the transient. High-end equipment will often have a configurable look ahead time.