It means the charge cord is not connecting correctly, or has a defect in the charger or cord. Try disconnecting each piece, then reattaching them. If that doesn't work I would try a different cord, then charger if that doesn't work.
I get it when using my S10 charger and also using my phone while charging. The note 10 charger is stronger than the S10. When youre using your phone the charger output is intentionally low and your app may be draining your phone faster than it can charge. If you lock your phone instead of using it while charging it shouldn't happen. That said, even with the message the drain should be minimal. Even without the message this phone will barely charge with the screen on. It mostly just maintains battery level until you lock the device.