The Action Phase occurs third in the turn sequence. During your Action Phase, you can play an Action card or play an Action entry on a Gear card you have equipped. This is generally where you attack targets, though other Actions exist as well.
Hostile Actions are considered attacks and permit a Reaction play from the target. Reactions include means of defense or countering, often played in an attempt to cause the Action to fail. If the target cannot play a Reaction for whatever reason (such as if they have none available or as a result of conditions), the Action automatically succeeds.
For more information, see Player's Guide Chapter 3: Scenarios.