An aggregator is a person or entity who gathers things together and then either displays them to others or utilizes what they have gathered for a purpose.
Data aggregators collect data from multiple points and sources and present them to users in a format that they find intuitive and easy to explore and/or use. Built on Cardano is an excellent example of a data aggregator, but data aggregators can also include those that aggregate news stories, as well as those who collect and display blockchain data in interesting and informative ways.
Data aggregation is a common thing in any and every industry and is a fundamental element of how many blockchain protocols work. The projects tagged as data aggregators on Built on Cardano are those who aggregate data as the main focus of their project i.e. displaying the data they collect to their users.