When measuring a single input-single output approximately linear system, such as a loudspeaker, the impulse response is defined as the output signal resulting from a short pulse input signal. The input signal pulse should theoretically be infinite short (a Dirac pulse).
The impulse response completely describes a linear system, it is quite intuitive (e.g. a hand-clap in a room will give the impulse response of the room), and the phase and magnitude frequency response can be calculated from the impulse response. This is partly the reason why WinMLS save the raw data in time domain as an impulse response.