Generally speaking, a preposition has a basic meaning (or perhaps a few), but there are contexts in which a preposition may have no meaning at all (or very little, anyway).
For example, in the sentence 'He lives in the Russian Federation', the word in has its basic meaning, but in the sentence 'I am interested in the Russian Federation' the word in has no meaning at all. In this latter case the phrase 'the Russian Federation' does not mean 'located in the Russian Federation'; the word in is used here simply to join the word interested with the noun it is used with. In grammatical terms we say that the word 'interested' in English governs a prepositional phrase consisting of the preposition 'in' plus objective case.
In Russian the same thing is true. For example, in the sentence Ћн живёт у Ивђна Петрћвича 'He lives at Ivan Petrovich's place', the preposition у has its basic meaning ('at a place associated with'), but in the sentence Ћн попросќл икрџ у Ивђна Петрћвича 'He asked Ivan Petrovich for the caviar (or 'He requested the caviar of Ivan Petrovich') the preposition means nothing at all; it is used here simply to join the verb попросќть with the noun it is used with — the verb попросќть governs у + Genitive. You learn how to use this particular у when you learn the word попросќть, not when you learn the basic meaning of у.