Quick answer
The light fitting or its wiring has a fault — usually a damaged cable, failed lamp, or moisture in an outdoor fitting. Switch the light off at the wall and call an electrician.
If your RCD trips every time you switch on a particular light, the fault is in that light fitting or the cable feeding it. Common causes are a damaged cable in the wall or ceiling, a failed lamp that has tracked internally to its fitting, moisture in a bathroom or outdoor light, or a poor connection that arcs when load is applied. Switch off that light at the wall switch and do not use it until an electrician has checked it. If it is a bathroom or outdoor light, moisture ingress is the most likely cause.
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