Cydia icon on my iPhone and other questions?

I had my iPhone fixed recently. I got it wet and had it in rice for several days, but it still had some problems. Someone I have found fixed it for me. Now I have Cydia icon on the phone. 1. What does it mean? Is it useful to me? 2. Is it used to unlock the phone and does it now mean that my iPhone is unlocked? 3. I did drop the phone in water - fully submerged - is it true that Apple can really tell if that has happened? Is there truly an indicator on the phone to show that? BTW - the phone is now fully operational. I am just curious on all above. THANKS A LOT!!!