What is a Zcash founder reward?

There is a reward for Zcash founders, developers and investors included in every mined block. The reward represents 20 % of the block reward (currently 2.5 ZEC) and it's deducted from every block value. The rest of the rewards (80 % of the block reward + transaction fees) goes to the miners.

For more details read this:  https://blog.z.cash/continued-funding-and-transparency/.