Prepositions are relationship words. They link nouns, pronouns, verbs and phrases together in a sentence. Without prepositions, it would be impossible to communicate. For example, would you understand a sentence that read "I want go the mall get a dress prom"? Perhaps you might be able to decipher it, but it certainly wouldn't be fun.
The missing words in that example are prepositions (to, and, for) and there are many more prepositions you can use too. Let's familiarize ourselves with a list of common prepositions.