Takže takéto sračky niekto považuje za správnu metódu učenia?...

Shoes that don't fit properly are bad ...
You should eat a diet that contains vitamins and ...
Sunscreen is important for people that have fair skin.
Junk food is food that isn't good for you.
People that don't get enough exercise often feel tired.

Relative clauses use "that", "which", "who" (most commonly, generally) to provide additional information. A sentence can have more clauses, if-clause, relative clause, that can be extracted into another (or other) separate sentence(s).

This task is funny. You are doing it. - two sentences
This task that you are doing is funny. - one sentence, two clauses

- by using "that" - we inform the listener/reader about the relation between one clause and the relative clause (they relate -> relative)
- do not confuse with "I said that you should come." or "I don't know that man."