What does react mean

It depends on which which instance you're using it in.

The general definition of "react" is : "respond or behave in a particular way in response to something"

It also depends on if you are using this scientifically or not, which in that case would mean a scientific reaction - where one substance reacts to another.

respond or behave in a particular way in response to something.

"he reacted angrily to the news of his dismissal"

synonyms: behave, act, take it, conduct oneself, proceed; More

respond with hostility, opposition, or a contrary course of action to.

"they reacted against the elite art music of their time"

(of a person) suffer from adverse physiological effects after ingesting, breathing, or touching a substance.

"many babies react to soy-based formula