1. Form and meaning
Relative clauses add detail about a noun or an entire statement. The relative pronoun matches the antecedent in gender and number, but its case depends on its role inside the relative clause. The conjugated verb goes to the end and commas mark the clause. Was often follows alles/etwas/nichts or refers to a whole statement; wo can refer to a place.
When you study this pattern, identify its communicative purpose first. Then check the verb form, case and word order. German clauses are easier to control when you learn the connector together with the sentence pattern it requires.