Prepositions are short words (on, in, to) that usually stand in front of nouns (sometimes also in front of gerund verbs). Even advanced learners of English find prepositions difficult, as a 1:1 translation is usually not possible. One preposition in your native language might have several translations depending on the situation. There are hardly any rules as to when to use which preposition. The only way to learn prepositions is looking them up in a dictionary, reading a lot in English and learning useful phrases off by heart.