This question was asked in Quora and I have answered this question in Quora, shared in Twitter and additionally I am also submitting the same answer to this question as a backup in Steem blockchain.
The Steem Blockchain follows a DPoS mechanism of consensus with 21 witnesses. The witnesses run the nodes and produce the blocks and then validate it & add it to the blockchain. In Steem blockchain, the block production time is 3 seconds, so every three seconds a new block is created. The "Head block" in Steem blockchain is a freshly created block in the Blockchain which is the latest & most recent block.
An "irreversible block" is the one that is validated by most of the witnesses and which can not be reversed or modified without a hard fork.
In order to check the block number of "Head block" and the "Last irreversible block", visit- Steemd.com and check the right margin of the page where it is displayed.
The difference between a Head block and an irreversible block is around 45 seconds.
Thank you.