It includes any Resistance check related to your immunity, as described by the power's description:
You are immune to certain effects, automatically succeeding on any resistance check against them.
So if your character has to make a Fortitude check to avoid getting poisoned or diseased, you automatically succeed that check and avoid any damage or penalty associated with it. Similarly, with 30 ranks, you become immune to anything that asks for a Fortitude or Will check.
On a smaller and less subjective scale, you can obtain immunity to certain damage types, like cold, electricity, fire or even magic. Being immune to fire, for instance, would automatically avoid any damage caused by fire sources, be them mundane or of superpower origin.
Keep in mind that a Resistance Check is a check to avoid the effects of powers and other sources of damage or negative effects on your character (pg 16):
A resistance check is an attempt to resist different effects, ranging from damage and injury to traps, poisons, and various power effects. A resistance check is a d20 roll + the appropriate defense (typically Dodge, Fortitude, Toughness, or Will).
There is a note on page 113 discussing the problems with Immunity and how the GM can fix it:
Under the Hood: Immunity
There are characters in comic books flatly immune to certain things. Immunity is intended to provide this option in Mutants & Masterminds. It’s simpler at some point to say a character is immune to something than it is to bother rolling dice. Immunity also encourages creativity: if you can’t overcome a foe just by hitting him, what then? Encourage players to use tactics, cleverness, power stunts, and hero points to deal with foes immune to their more conventional attacks.
If you find Immunity—especially broad immunities at higher ranks—a problem in your game, feel free to restrict it (perhaps to no more than 10 ranks) or eliminate it altogether, replacing it with Protection and defense bonuses with appropriate power modifiers.