Here you have two useful summaries of relative clauses if you need to have the explanation in Spanish.
1. This slide presentation gives you a very good introduction.
Oraciones de relativo en inglés from Vicent Pérez
2. In this link you have a complete description of what they are and how to use them with examples and exercises at the end.
https://english.lingolia.com/es/gramatica/estructura-oracion/oraciones-de-relativo
