@Frappucinoo I will try and explain you in simple words.

1- Bitcointalk accounts are often hacked.
2- To ensure you didn't hacked the account, you have to prove that you are the owner of the account.
3- How to prove that? By signing a message from a BTC address you used in past. Signing a message is easy if you have private key of the BTC address and the guide was shared a few posts back by hilariousandco