A hard fork (or hardfork) is a permanent divergence in the blockchain, which occurs when non-upgraded nodes can’t validate blocks created by upgraded nodes that follow newer consensus rules. Cryptocurrencies after hardfork share a transaction history up to a certain time and date.
The first intentional hard fork splitting bitcoin happened on 1 August 2017, resulting in the creation of Bitcoin Cash. Other coin split created altcoins such as Bitcoin Gold or Bitcoin Private. Owner of bitcoins automatically gets the newly created coin via coin split so if you owned bitcoins during bitcoin hard fork coin split now you also own the new coin (eg. Bitcoin Cash). If owner wants to claim his it has to be implemented in wallet.
Trezor Wallet implemented claiming tools for Bitcoin Cash as well as for Bitcoin Gold(BCH,BTG) where it is possible to claim these coins for users who owned Bitcoin funds on addresses when these hard forks happened.