A preposition shows the relationship between nouns in a sentence.
The defendant will appeal against the decision.
In this sentence the preposition, ‘against’, is a link between the nouns, ‘defendant’ and ‘decision’.
Prepositions can also indicate the position of one object in relation to another.
The boy walked over the bridge.
The preposition, ‘over’, indicates the position of the noun, ‘boy’, to the other noun, ‘bridge’.