Reactivity in psychology is a way of describing how a person reacts or responds to different things going on around them. It's kind of like how your body reacts when you see a spider or a bug.
When someone is talking about reactivity in psychology, they usually mean how quickly or strongly someone reacts to different situations. For example, if something really scary happens, some people might jump and scream really loudly, while others might just calmly walk away.
Reactivity can also refer to someone's emotional responses. For example, if you're feeling sad, you might cry or feel tired, while if you're feeling happy, you might smile or laugh a lot.
Overall, reactivity is just a fancy word that basically means how someone reacts or responds to different things, whether it's emotionally or physically. And just like everyone is different, everyone's reactivity is different too!