Icing is the build-up of ice on the propellers and airframe when flying in visible moisture — fog, cloud, drizzle — at temperatures around or below 0 °C. The most dangerous spot is the propellers themselves: thin blades spinning fast through moist air.
On small drones, ice on the props destroys lift and control very quickly — there is practically no margin. The rule is simple: do not fly in icing conditions at all, and at the first sign — vibration, an unexpected rise in power draw, ice on the airframe — land immediately.