A so-called fork is a split in the Bitcoin network.
The Bitcoin network consists of a number of miners that secure the network by providing their computing power to solve mathematical calculations. These calculations enable the consistent addition of new blocks to the blockchain to confirm transactions.
A split of the network occurs if a number of miners decide to solve problems based on a new protocol and results in two blocks being added to the blockchain instead of only one block.
Subsequently, the chain of blocks is split and two different chains result, e.g. the Bitcoin blockchain and the Bitcoin Cash blockchain. Miners are free to decide which blockchain they want to support with their computing power.