Starting with OctoPrint 1.3.10, the bundled "Pi Support Plugin" (only active when running on a Raspbery Pi) will regularly check if your Pi has detected any underlying issues via
This command can detect whether your Pi is currently throttled, or has been in the past, either due to undervoltage or overheating.
The Pi Support Plugin evaluates this information and displays corresponding icons in your navbar in case such a situation is detected:
In the above example, the Pi has detected both undervoltage and over heating. Clicking on the symbols will lead you to this FAQ entry. Hovering over the symbols will give you further information:
In this popover, detected issues will be highlighted in red (and - if a current issue - also blink just like in the navbar).
If your Pi is reporting undervoltage, you need to change the way you are powering it. Do not use some cheap phone charger you still have lying around, use a proper power supply that provides 5V and at least 2A current reliably. Also make sure that the cable from your power supply to the Pi is up to the task - there are cables out there that are not allowing needed voltage to actually arrive at your Pi due to too high internal resistance. Use a good quality thick cable, not some flimsy thing you still had somewhere in a drawer.
If in doubt, the official Raspberry Pi power supplies are a good option.
If your Pi is reporting that it's overheating, you need to provide it with better cooling. Make sure that air can circulate freely around it, and possibly add some cooling sinks to the chips.
In any case, if you get a report on any current issue or any issue observed since boot up, be sure to look into them - the Raspberry Pi is known to behave erratically especially in undervoltage situations and cause all kinds of issues, including communication issues with your 3d printer that can and will ruin your prints. If things are set up well, you really should never see these symbols pop up.