There are thousands of Miners in Bitcoin Blockchain who compete against each other to mine a Block so that they can win a mining reward of 6.25 Bitcoins. There is a possibility that two miners may solve the mining puzzle at the same time and broadcast the Block to other miners. For example, Miner ‘A’ and Miner ‘B’ have solved the puzzle at the same time. Due to the propagation speed of the Blocks across the network, a few miners may receive the Block from miner ‘A’ first and other miners will receive the block from miner ‘B’ first which will create a temporary conflict between the miners.
Due to this conflict, a few miners are building their next Block based on Miner ‘A”s Block and others are extending the chain based on Miner ‘B”s Block.
A simple solution to this problem is, whosoever is able to mine the next block whole network will consider that chain as a valid chain. For example, if the next block is mined on Miner B’s block that chain will be considered a valid long chain. Other miners who were building on Miner A’s block will abandon that block and adopt Miner B’s block as a valid chain.