A miner is a service provider to the network of other users of a cryptocurrency.
Basically, the miner ensures that every transaction made in the network is entered in
a so-called distributed ledger. All miners agree on a single valid ledger.
So no user can fake transactions, since the information on them can not be changed afterwards, as it is stored in a publicly accessible ledger.
For this service, one miner receives a block reward in the respective cryptocurrency after each block.