A multiple loop occurs when a player takes a meaningful action in the same real time environment as other players, and which requires action of other players to determine a win. In simpler terms this means most sports, online first person shooters and cooperative modes in action games.
Multiple loops are are both synchronous and contemporal. Play based on multiple loops is often described as multiplayer. In older times this used to mean several players playing on the same console, or over a PC local network. However widespread fast broadband has extended the range of multiplayer considerably.
Multiplayer games (or modes) often have smaller audiences compared to single or parallel games because of the need to organise groups of players, but they also tend to form some of the most loyal long term fan bases. They also work very well as spectator sports.