"起缓解作用之物" can be translated into English as "something that provides relief" or, more formally, "a relieving agent/substance" (depending on the context).
For example:
"This medicine acts as a relieving agent for pain." （这种药能起到缓解疼痛的作用。）
"A warm bath can be something that provides relief for stress." （洗个热水澡能缓解压力。）