They are often used to describe the home row keys, the row of keys your fingers rest on when you're typing.
This phrase is often used:
* In typing exercises: People use these keys to practice finger placement and learn proper typing technique.
* As a shorthand: Some people might use "SDFGHJKL" to refer to typing in general, or the specific keys used for a particular task.
Let me know if you have any other keyboard-related questions!