Every Bitcoin block value consists of the block reward (new coins created) and the fees from transactions contained in the block. The block reward is hard coded for each block and its value is halved at intervals of 210,000 blocks (approximately once every four years).
When Bitcoin was created in 2009, the initial reward was 50 BTC. Now the reward is 12.5 BTC and it is going to be cut in half to 6.25 BTC in spring 2020 (see countdown). The halving process will continue until the final supply of coins - slightly less than 21 million - is reached.