Okay kiddo, have you ever heard someone say "who" or "that" when they're talking about someone or something else? That's what we call a relativizer! It's a fancy word that means it helps us understand more about something else in a sentence.
Let's say your big sister told you about a cat she saw in a park. She said "I saw a cat." Okay, cool, but we don't know anything else about the cat. But if she said "I saw a cat that was orange and stripes" then we know more about the cat she saw! "That" is the relativizer here. It's telling us that there's more information coming about the cat.
Other examples of relativizers are "who," "whom," "whose," "which," and "where." They all help us understand more about something or someone in a sentence. So next time you hear one of those words, remember that it's a relativizer and it's helping you learn more!