Night is a phase in the game that comes between Day phases. All players with Night Actions (all of which are power roles or scum) may privately send them in to the moderator during this time; however, many moderators will allow Night Actions to be sent early as well.
Nights usually have a deadline attached. Actions that are not sent in by that time are forfeited, or in the case of Compulsive actions, randomized. Occasionally circumstances will force the moderator to wait until everyone has submitted their choice before going to Day, but this is thankfully rare.