Let me explain with an example.
Suppose a person A having a bitcoin wants to sell it and transfer the money to his account. He authenticates his identity and authorized the transfer. Now the question is, who verifies the transfer?
When you're transferring money to your personal account the transfer is validated by the bank which is a legal authorized party in the transaction. In a bitcoin transfer, millions of bitcoin holders validate your transfer. So instead of one authority validating a transfer, you now have multiple people validating a transfer.
Whoever is the first person to validate a transfer gets one bitcoin as reward. This exercise of validating transfers is called bitcoin mining.