The activist decided to stand his ground against the unjust policy.

stand 在这里是动词，意思是"站立、坚持"。 his 是代词，指代"他的"，在这里指代活动家自己的立场。 ground 本义是"地面"，引申为"立场、基础"。 stand his ground 是一个固定短语，意思是"坚持自己的立场、不退让"。