A preposition is a word that connects a noun, pronoun or noun phrase to other parts of a clause.

Example
‘I insist on it' and ‘We went from here to there in 10 days'.

In the classroom
Dependent prepositions - prepositions which always go with certain language - cause problems even for advanced level learners. Memorisation and controlled practice can help, but perhaps the best way of gaining control over prepositions is through a lot of exposure to authentic language.