Hi Fizban
Finrot is a word commonly used to describe a condition that looks like a fish's fins and/or tail has been eaten away, torn or otherwise damaged. It's a bacterial infection which might or might not show other symptoms as well.
Some of these other symptoms include, but aren't limited to: ulcerations on the body (red or pink sore spots), fuzzy white stuff that looks like fungus on the mouth or body, streaks of red anywhere on the fish, or gasping for air at the top of the tank.
The most common cause is inadequate or insufficient water changes and bottom vacuuming, but temperature fluctuations, overcrowding, low oxygen content in the water, aggressive tankmates, high nitrites or nitrates in the water, overfeeding, or any other thing that causes undue stress to the fish.
If you are in doubt, do a very good tank cleaning and watch the fish carefully.