Prepositions are words that together with grammatical cases of nouns, pronouns,and numerals help to express relationships between different words: сидеть за столом - sit at a table, прилететь в Анапу - to arrive to Anapa city, подъехать к офису - to stop by the office.
Prepositions are not independent parts of speech, rather they belong to the words they're used in the sentence. Most prepositions are typical of certain cases: Я буду скучать по тебе (I'll be missing you) - Dative case, Я пошёл с ним (I went with him) - Instrumental case etc.

