First thing to know is ‘i’ is your count. So you start at i=0 on the first day of bitcoin. 2 to the power of zero is 1 (can test on calculator). So there are 50 bitcoins each block for 210,000 blocks. At which point the i clicks up to 1 and you have your first halving (2 to the power of 1 is 2 and therefore you have 50 divided by 2 for your next blocks = 25 Bitcoin for 210,000 blocks). And so on…