The Bitcoin whitepaper states:
Nodes always treat the longest blockchain as the truth and continue to expand on it.
All miners should work on the longest blockchain because it helps to maintain the uniqueness of the blockchain ledger. If the Bitcoin sent to you is not recorded on the longest blockchain, You could possibly lose it.
What is “the longest blockchain”?
Since every miner around the world is mining simultaneously, it’s likely that two miners arrive at the right answer at the same time. The blockchain will fork, and other miners may choose to keep mining on one of the fork to extend the blockchain.
That’s why we usually require 6 block confirmations after the Bitcoin transaction is processed. We do that to ensure that the miners won’t return to the other forked chain, and then we can consider the transaction to be successful.