Definition
The version field in the block header serves dual purposes. Originally it simply indicated the block format version, but since BIP 9, it has been repurposed for signaling support for soft fork proposals. Individual bits within the version field can indicate miner support for specific protocol upgrades.
When enough miners signal support for an upgrade (typically 95% of blocks in a retarget period), the upgrade activates. This mechanism was used for the SegWit activation, among other upgrades.
In Simple Terms
A block header field that indicates the block format version and signals support for protocol upgrades.
A 4-byte field in the block header indicating the block version and signaling support for protocol upgrades via version bits.