"What's an impulse response?", you ask. Only your key to unleashing killer guitar tones at any volume. Finally, you can give the neighbors some other reason to call the cops.
For the technical among us, an impulse response (or "IR") realistically captures the characteristics, both in frequency and time, of an entire signal chain, including the sampled speaker, the microphone, the room it's in, the power amp, preamp even the A/D converters.
Put another way, it'll capture the unique frequency curve of the speaker, any phase smearing inherent in the mechanical operation of the speaker, any cabinet resonance, the frequency response and resonance of the mic, the sound of the room, and any frequency or phase related coloration introduced by the power amp, preamp, and converters and apply that captured sound to any audio you pass through it... or so our even geekier friends tell us.