EF = Entity Framework. (1)

It's an ORM (Object-rational Mapper) for querying SQL databases into objects. Somewhat the same space as "Hibernate" in Java. (2)

Some prefer e.g. Dapper (3) as it's much more light-weight. EF is very much a big thing, but some people need and want that.

1) https://docs.microsoft.com/en-us/ef/

2) https://hibernate.org/

3) https://dapper-tutorial.net/

EF is ORM, basically wrapper over communication with database

It really, really speeds up the work, but there's a lot of to learn about how does it work, its limitations and conventions