Put really simply, a blockchain is a chain of blocks (duh) where each block consists of several transactions. Each block is connected to the block before it and thus they form a chain. When you calculate all the transaction over all the blocks in a blockchain, you get the balance in each address. You can't change a block in between and re-insert it to the chain because each block is connected to the previous block. And you can't just add funds to yourself because the system is decentralized, which means everyone who has a node has a say over what next block will be, and the longest blockchain is usually the approved one. So trying to trick the system won't work unless you have the ownership of over the half the nodes. There are all kinds of methods on how to confirm if a block or a blockchain is valid, such as proof of work and proof of stake (simply put, calculating/guessing the number that decrypts the block), and changing the difficulty of this confirmation process sets the time required to decrypt/mine a block. Too low and the system becomes less secure, too much and the system stops working.

There are tons of videos on Youtube and other platforms that you can watch. I believe others have already linked a few.

Good luck and hope you don't get fired. Don't cheat next time