Hi @swkim109,
The best description of a chain reorganization I could find is from an Ethereum blog post by Gavin Wood:
https://blog.ethereum.org/2015/08/08/chain-reorganisation-depth-expectations/
Chain reorganisations happen when a node on the Ethereum network (one which could belong to you, me, an exchange, a miner, whoever) realises that what it thought was the canonical chain turned out not to be. When this happens, the transactions in the latter part of its chain (i.e. the most recent transactions) are reverted and rather the transactions in the newer replacement are executed.With Ethereum having a short target block time of 15s, this actually happens naturally rather often. Because it takes time for the blocks to percolate through the network, it’s easy for different parts of the network to have a different final block (or two, or perhaps even three) in normal operation since the miners often come up with them at roughly the same time. This is what we might call ephemeral forking. Indeed, many of the ommers (né uncles) that you see in stats.ethdev.com were once assumed by some nodes to be the final block in canonical chain.
If anyone in the community has a better description, then feel free to add it.