What is “Weather and Chaff” information provided to pilots by ATC?
In the Pilot/Controller Glossary, this is the definition of &quot;Chaff&quot; and would be its meaning in the context of &quot;Additional Services&quot; referenced in your question.

CHAFF- Thin, narrow metallic reflectors of various lengths and
frequency responses, used to reflect radar energy. These reflectors,
when dropped from aircraft and allowed to drift downward, result in
large targets on the radar display.

On some radar equipment chaff may appear similar to areas of precipitation.