The word preposition can easily be understood by listing some examples: in, on, under, by, for, from, with, during. Prepositions are the (usually) short words that precede nouns or pronouns, and give information about where (on the table), when (in April), etc.
What can be very difficult for learners of English, however, is to know which preposition they need in the sentences they say or write. This is because there is often no correspondence between the preposition they use in their language and the one needed in English*.
* For example: German has the same preposition as English: in. But there are many occasions when English uses in, and German uses a different preposition, or vice versa. English children play in the street, German children auf der Strasse (on the street). Germans can be good in languages (gut in Sprachen), whereas the English are good at them. English people travel by bus; Germans travel with the bus (mit dem Bus). In Germany, something belongs me (gehört mir), but in England it belongs to me.
Prepositions often follow other parts of speech as fixed phrases that have to be learned case-by-case: interested in, angry about, to belong to, etc. It is clear, therefore, that the learner who wants to use English prepositions correctly will often need to consult a good dictionary.
An effective way to learn the most common uses of the most common prepositions is to do exercises in a grammar book such as Murphy's English Grammar in Use. Alternatively, you could: