What they are:

https://www.coindesk.com/short-guide-bitcoin-forks-explained/

Are they safe:

They don't affect you or your coins in any way. The complications start when you start claiming them. Once a fork is launched, you're likely to find a lot of wallets, in which you will need to import your Bitcoin private key to gain a corresponding amount of the forked coin. Fake wallets are going to take your private key, and steal any Bitcoins within. Make sure you only use the official wallet for the fork and empty the Bitcoin address whose private key you will import beforehand.

How to earn:

Once you claim them, you hold then sell, or you sell immediately. You earn from them just like you earn from any other altcoin, except you got them for free.