Blockchain Fork means that the blockchain of Bitcoin and other cryptocurrencies, which is connected in a single chain, branches off in the middle.
For example, in Bitcoin (BTC), blocks are generated by mining, but there are cases where multiple blocks are generated at about the same time, either intentionally or accidentally.
In this case, the multiple blocks generated branch off as separate chains. This phenomenon of a blockchain branching off is called forking.
Soft Forks and Hard Forks
Forks can be broadly classified into two types: Soft Forks and Hard Forks.
A Soft Fork is a divergence that occurs when blockchain specifications are updated, and in most cases, the diverging chains will eventually converge into one.
A Hard Fork is a backward incompatible update where the divergent chains do not converge and a different cryptocurrency is created from the original one. A typical example of this is Bitcoin Cash (BCH), which is a branch of Bitcoin (BTC).