A consensus breaking change is one that requires everyone to upgrade their software client so that consensus is maintained. For example: updating the block size in Bitcoin. If larger blocks are implemented in Bitcoin, everyone must update their client because old software basically cannot process larger blocks. Such changes are hardforks and those who do not update their software cannot be synchronized with the blockchain.

A consensus breaking change is one that requires everyone to upgrade their software client so that consensus is maintained. For example: updating the block size in Bitcoin. If larger blocks are implemented in Bitcoin, everyone must update their client because old software basically cannot process larger blocks. Such changes are hardforks and those who do not update their software cannot be synchronized with the blockchain.