A hard fork is when all the nodes and miners adopts a new rule at a certain point on the blockchain. The rule change can be anything, Inflation change, major code upgrades, etc... This means technically 2 new blockchains will be formed. Usually the old chain will be abandoned once the nodes and miners stopped maintaining it - when they jump to the new chain.

Typically, hard forks are denoted by block number, as you can see in example such as Bitcoin , Litecoin and Bitcoin Cash. Meaning "Guys, we will all set the new rule at XXXX block number" so all nodes and miners are synchronized. (Note: halving is not hard fork, just an example in this case)

But for Steem, a DateTime is used to make it easier to the Steem community to understand approximately when the hard fork will happen.

As mentioned, technically 2 new chain will form - one that is hard forked and the other that still uses the original rule. Witness nodes that have not upgraded to Version 0.21 will run a version of Steem that did not hardfork. Depending if the old chain is maintained, it could be possible that 2 new chains be formed but in this case it is very unlikely because most Dapps will run on hard fork 21.