Relational Databases (also refered to as Relational Database Management Systems–RDMS) are a class of databases that store data in different tables. Their name is derived from the fact that it is possible to reference a table in another one and thus establish relations.
How does that look like in practice? Let’s take the artist-album relation as an example. That’s a one-to-many relation because one artist can record several album. Typical RDMS handle that with ease, using a reference in album table back to an artist’s unique identifier.