Flashing
Web pages do not contain anything that flashes more than three times in any one second period [...]
Flashing content is a common trigger for seizures. To avoid this, ensure that no content flashes more than three times in any one second period.
It can also prove to be very distracting and detract from the overall user experience.
There are some more in depth rules that can be found on the WCAG 2.3.1 (opens in a new tab) page but in general, avoid flashing content.
How to fix
Largely this is likely to be a design question, but this can happen due to flickering content that shouldn't be flickering and due to content that may be refreshing too often.
In general, this is just an important consideration to be aware of and to keep in the conversation when implementing new features.
Provide an option to stop flashing
However, if there is flashing content that must exist in the app for some reason, it is important to provide an option to stop the flashing. User's must be able to avoid, turn off, or adjust the flashing, and the flashing should probably be off by default.