Rubidium is a trace mineral that acts like potassium in the body. It is not an essential nutrient, and humans have no known biological need for it.
Few people take rubidium as a supplement. Practitioners of alternative medicine sometimes claim it supports mood or nerve signaling because of its chemical similarity to potassium. Rigorous clinical trials do not back those ideas.
Mainstream health authorities do not recommend rubidium supplements. Too little is known about their safety or whether they do anything at all.