An aggregation (often called a rollup in no-code tools) is a summary value calculated from a group of related records through a relation - a sum, average, count, minimum or maximum. A typical example: a client record can aggregate the total of all their invoices, or a project can aggregate the count of tasks still open.
The difference from a plain formula is that a formula usually works with values within a single record, while an aggregation runs across an entire group of related records at once - dozens of invoices, hundreds of tasks - and returns one number.
Aggregations are the foundation of reporting in database apps: a job's status, a client's margin or a team's workload can be calculated from the lowest level of data (individual invoices, individual hours logged) upward, instead of being tallied by hand in a separate table.
Related: Reporting via view columns and formulas, The agency that knows what it actually earned on every job, What is a formula.