Monopropellant is a resource that is used by RCS thrusters. It is stored in separate tanks from liquid fuel or oxidizer, and may be transferred around a vessel. Stored monopropellant is available to every thruster on a vessel. As of version 0.23.5[outdated], containers on earlier stages are drained before containers on later stages.
Monopropellant is also used by the jetpacks worn by Kerbals on EVA. Each kerbonaut can carry 5 liters, which is enough for about 600 m/s of Δv. The jetpack is resupplied when the kerbonaut enters a command module, but as of version 0.23.5[outdated] this action does not remove any monopropellant from the craft's supply.