Bitcoin are mined in units called "blocks." As of the time of writing, the reward for completing a block is 12.5 Bitcoin. At today's price of about $8000 per Bitcoin, this means you'd earn (12.5 x 8000)=$100,000.
When Bitcoin was first mined in 2009, mining one block would earn you 50 BTC. In 2012, this was halved to 25 BTC. in 2016, this was halved to the current level of 12.5 BTC. In 2020 or so, the reward size will be halved again to 6.25 BTC.
If you want to keep track of precisely when these halvings will occur, you can consult the Bitcoin Clock, which updates this information in real time.