what is external force in rigid body?

I read the linked pages below, but I didn’t understand what external force was like or how to use it. What else should I look for and read? Who wants to explain it to me?
pls, help me!

Rigid Body | 언리얼 엔진 문서 (unrealengine.com)

애니메이션 블루프린트 리지드 바디 인 언리얼 엔진 | 언리얼 엔진 5.0 문서 (unrealengine.com)