Bitcoin transactions aren't reversible..
Due to lack of awareness about crypto, I'm still confuse at this topic. Does it mean something like this,
I send 1BTC to address "B" from Address "A". Next I try to send back that 1BTC to address "A" from address "B". Is it an impossible thing?
Simply means, when you mistakenly sent transaction to a wrong or unknown address once transactions are confirmed there's no way for you to cancel or get it back on your end.
Based on your example, if you are the owner of both address A and B of course you can send it back. However, beong irreversible means, you cannot do this task if ever let's say wallet B is another parties address, there's no way for you (address A) to cancel or reverse a transaction on your end once the transaction is confirmed. This is to avoid fraud, bitcoin is created as a form of payment. If payments can be reversed, users can just get their money back after a transaction, which is not good.