An aggregator is a technology platform that connects casinos to games from many providers through a single integration. Rather than linking separately to dozens of studios, an operator connects once and chooses games from the aggregator's catalogue.
Aggregators handle the technical plumbing: game launch, wallet and transaction messages, reporting and sometimes promotional tools such as tournaments. They also help manage which games are available in which markets, so only GB-certified versions reach GB players.
For players, an aggregator is mostly invisible. It explains why many casinos have very similar game libraries, and why the same game behaves the same wherever you play it, apart from the RTP version the casino has chosen.