A relation is a connection between two entities in a database - like a client and their jobs, or a job and the invoices that belong to it. Thanks to a relation, the system knows what belongs to what, and can compute summaries, filter and display connected data in one place.
The most common type of relation in a business database is "one to many" - one client can have many jobs, but each job belongs to only one client. There are also "many to many" relations (like employees and the projects they work on) or "one to one" for edge cases.
Without relations, every table would live in isolation, much like separate sheets in a spreadsheet - and any summary across the data (how much a company earned from a client, the status of every job for one salesperson) would have to be calculated by hand.
Related: A data model that grows with the team, Reporting via view columns and formulas, How to connect documents, databases and chat into one workspace.