By default, the main actor can be moved around in a map by the player. Events can be configured to take away movement control from the player so that a scripted event or cutscene can take place without interference from the player.
Parties are formed and consist of multiple actors which the player have access to. Party members can be changed by use of event commands, so that a different group of actors can be controlled by a player.
A party can only consist of a limited amount of actors, usually four, but the limit is eight in RPG Maker MV.