The primary actions a character takes become less effective the more times they use those actions in a round. The basic example for this is attacks: after using an attack action, you receive a cumulative -5 penalty on further attack actions. This rule extends the concept: if you cast a spell using 2 actions then make an attack, the attack should still suffer a penalty.
Here is the basic idea: after using any action that doesn’t have the Minor keyword, you receive 1 MAP (multi-action penalty), unless noted otherwise in the ability. The effect of these multi-action penalties varies. For attack rolls, each MAP gives a -5 attack (varies with weapon type). For spells, it might reduce the effective level. For bolstering, it might reduce the number of targets, etc.