search is your friend....there's loads of threads on this here.
In context for here, when people are talking about impulses it's usually in regards to speaker cabs where someone has captured how sound is "effected" by a mic'ed cab.....poorly explained, but I'm trying
....it's bloody easy to use them but tough to explain.
In simple practice you're supposed to be able to play a amp or POD without their corresponding speaker cabs (or cab sims), slap on the impulse response for a cabinet and have it sound like the real thing (or a "super-POD"). It's a practical way of getting the sound of a rockin'ly loud cab if you don't have the means of doing so for whatever reason (neighbours, gear, etc). It's great for POD's and a good impulse will sound better than the cab sims on the POD itself. Some of the guys here share their impulses (Splat88, Kazrog, Keiffer, etc) and they are excellent for that purpose.
This "sound" of the cab is usually captured via running a test-tone through the cabinet and then recording that (via a mic of choice) into wav format. Then you can load that into a plugin such as SIR or Pristine Space. Slap that puppy on the un-cabbed amp or POD tone with the cab sim turned off, and it's rock time!
Impulses can also be used to recreate acoustic spaces, just like reverbs. It's just not a digital representation of a space, but the sound of the "actual" space. I saw some muppet capture the sound of his mouth the other day....by setting off a starter's pistol in it!
uke: 
