When we feel overstimulated, we might feel stressed out, upset, or overly tired in public places due to the number of people or amount of noise. That's caused by having too much sensory information coming into our brains at once.
When we're stimulated, we're feeling active, interested, and excited. However, it's easy for that to get too intense, and then we might become overstimulated and start feeling overwhelmed or out of control. Other things can become overstimulated, too. Certain chemicals, for example, might overstimulate the production of other chemicals, causing an unusual increase or spike.