I can try. The purpose of mining is to solve a puzzle. In Bitcoin one solution is expected every 10 min on average. This can then be used store transactions on

I can try. The purpose of mining is to solve a puzzle. In Bitcoin one solution is expected every 10 min on average. This can then be used store transactions on the blockchain in one block. To add to that block you need to solve a new puzzle and so on. You can reverse transactions if you recalculate the puzzle solutions. This can take time but if you have more mining power than the rest of the network you can win this race. This is called a 51% attack. So for example you deposit to an exchange and wait for that to confirm, in maybe 6 blocks. Meanwhile you create 7 blocks in secret yourself where you did not send to the exchange and release them to the world. The blockchain works by always considering the longest chain as the correct one, therefore overwriting or reversing the old transactions.