Compiling a material involves determining which Techniques are supported on the card on which the engine is currently running, and for fixed-function Passes within those Techniques, splitting the passes down where they contain more TextureUnitState instances than the current card has texture units.
This process is automatically done when the Material is loaded, but may be repeated if you make some procedural changes. By default, the engine will automatically split texture unit operations into multiple passes when the target hardware does not have enough texture units.
