Subjects and predicates are the essential, most basic building blocks of a sentence. They’re nothing fancy, just beginner-level grammar. Before you learn any of the more sophisticated grammatical concepts, you have to know what a subject and predicate are. Also, you must know what they are to avoid writing grammatical gaffes like sentence fragments, subject-verb disagreements, and dangling modifiers.
You can add things like subordinate clauses, attach them to other sentences to make compound sentences. They’re the steel scaffolding of grammar. Once you have a good, solid sentence, you can hang all sorts of shiny things on it and it will stand up straight and tall.
It’s very likely you already have an intuitive understanding of subjects and predicates but lack the proper vocabulary to describe them. In that case, this video will give you words to name things you already know in your gut.