When miners create new blocks, the rewards received in return for this work put into creation of new blocks is referred to as block rewards. Block rewards are new bitcoins generated after miners put in their hardware computing power into solving complex mathematical operations so as to complete previous blocks and create new ones. The network awards these bitcoins as rewards for miners that create new blocks successfully.
Block is a record for transactions taking place between two parties. If a sender makes payment to a receiver, the block stores this transaction and verifies it. It can accommodate up to 1MB size of transactions. Other cryptocurrencies also reward miners that use their hardware computing power in creating new blocks in the same manner as bitcoin mechanism.
There can only be 21million ever in supply. Hence, the block rewards decrease every four years halving the previous block rewards. The first bitcoin block rewards is 50bitcoins. As the block rewards decrease, this will create scarcity and makes it price goes up. For every block rewards created, it becomes the first transaction for new blocks.