Monopropellant is a resource that is used by most RCS thrusters along with the O-10 MonoPropellant Engine. It is stored in separate tanks from liquid fuel or oxidizer, and may be transferred around a vessel. Crew containing command modules also store a small quantity of monopropellant. Stored monopropellant is available to every thruster on a vessel. As of version version 0.23.5, containers on earlier stages are drained before containers on later stages.
Monopropellant is also generally assumed to be used by the jetpacks worn by Kerbals on EVA. Each kerbonaut can carry 5 units of "EVA Propellant", 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 The jetpack is resupplied when the kerbonaut enters a command module, but as of version 1.1.3[outdated] this action does not remove any monopropellant from the craft's supply.
Real world equivalent
In reality, most manned vehicles burn a hypergolic (self-igniting) mix of some hydrazine-bearing chemical and dinitrogen tetroxide in their RCS thrusters. Monopropellant systems are more common on satellites and space probes, such as the Mariner-Voyager series.