Condenser tumble dryers have a water tank or reservoir that fills up during the cycle. Most dryers will warn you when that tank is getting full, so you can empty it.
Some tumble dryers simply have a light with text like ‘Empty water’ next to it. Other models have a symbol that lights up.
The symbol for ‘Empty reservoir’ or ‘Empty water container’ is often:
A tray with a water drop above it
A tray with a wavy line in it