It means one of three things:
- The received signal strength is below 60%, or
- The noise level is above 10%, or
- The app has detected co-channel and adjacent channel interference.
When co-channel or adjacent channel interference is detected, the app will automatically recommend alternative channels if the app determines that switching to a different channel may reduce the level of interference from nearby networks.
In wireless networks, having a strong signal does not necessarily mean you have a good quality connection. Besides signal strength, you have to take into account noise and interference, hence the warning sign.