The signature proves the wallet is yours. It is valid for 30 days. It is not a transaction, it costs no fee and it moves no funds.
After you sign in, actions such as posting, following, voting, editing your profile and changing settings need no further prompt. Anything that moves money, deploys a coin or acts on-chain always asks your wallet again.
If you decline, you can keep browsing. Those actions then ask for a signature each time you use them.