A Night Action is any active role ability that happens during the Night phase of Mafia. The range of Night Actions vary depending on games, but the most common types are investigations, protections, roleblocks, and nightkills.
Night Actions give players the most concrete information gained in a Mafia game, apart from cardflips from executions and nightkills. For this reason, a few games include a Night 0 before the game starts to give certain players a little bit of information for Day 1.
Night Actions are presented differently depending on the moderator handling them. Many moderators include flavor when reporting night action results to players, although this is potentially dangerous for game balance as flavor has been used in the past to confirm players as Town- or scum-aligned. Some moderators include throwaway night actions that have no effect on the game to add uncertainty to it or to make it harder to identify genuinely vanilla players (ex: Ecco's France 942). Finally, failure reports are handled differently; some moderators tell power roles exactly how their Night Actions failed while others only say that they failed and still others do not give any information one way or the other (unless the Night Action necessitates a report, such as investigations). So while Night Actions are typically reliable and quite useful, their usefulness depends on the moderator handling them.