uPnL stands for Unrealized Profit and Loss.
An Unrealized Profit occurs when an asset is purchased and then rises in value but hasn't been sold:
An Unrealized Loss occurs when an asset is purchased and then falls in value but hasn't been sold:
There is another place on 3Commas where you can see the uPNL.
It's on My Deals page, and it's uPNL of active Bot Deals:
The number displays how much profit or loss ALL active trades currently have. In other words, this much loss (or less profit) you can get if you close all active deals at once at the current market price.