A group introduction or character introduction is when a player character is introduced by their player for the benefit of the gaming group. Group introductions take place after character creation, usually either at the start of the first session of the campaign in which the new character appears, or at the moment during the session when the character appears in the fiction.
While a player may play primarily in character, an introduction generally requires out-of-character narration, e.g. to describe the new character's appearance or other characteristics that can't be easily conveyed through acting.
Conventions vary between groups on whether the information presented during an introduction should include only what the other player characters will be immediately aware of, or whether a fuller description (e.g. including personality and motivation) is included as well. The latter can help other players know how their characters can best interact with the new character, improving group and party cohesion, and it can represent insight gleaned from quiet interactions that aren't played out (i.e. between scenes).
If all characters are being introduced at the same time (e.g. at the start of a campaign) and the group had a session zero, players will already understand each others' characters and therefore a simpler description is usually sufficient.