A cryptocurrnecy hard fork is a software upgrade that introduces a new rule to the network that isn't compatible with the older software. You can think of a hard fork as an expansion of the rules. A new rule that allows block size to be 2MB instead of 1MB would require a hard fork.
Nodes that continue running the old version of the software will see the new transactions as invalid. Switching over to the new chain and to continue to mine valid blocks, all of the nodes in the network need to upgrade to the new rules.
The Ethereum network will be undergoing a planned hard fork at block number 4.37mil (4,370,000), which will likely occur between 12:00 UTC and 13:00 UTC on Monday, October 16, 2017. The Ropsten test network underwent a hard fork on September 19th (UTC) at block number 1.7mil (1,700,000).
On approximately November 18th, when block 494,784 is mined, a hard fork enabling the bitcoin network to carry 2MB blocks is expected to occur. This follows the soft fork upgrade earlier this year that added Segregated Witness (SW) signatures to the network. Many refer to the combination of these upgrades as the New York Agreement (NYA) or SW2X, which was supported by a majority of the consumer facing industry, including Blockchain, as well as miners.