In a chemistry lab, a reagent is a substance you add on purpose because it will react, and that reaction tells you something — often that another substance is there, or how much of it there is. A school-book picture: you drop a reagent into a liquid, the colour changes, and that change is the answer. That “testing” use is the one learners meet first. Though actually, chemists also stretch reagent to mean almost any chemical they put into a reaction, not only the one that “shows” something; in that wider talk it sits near reactant (a thing that gets used up in the reaction). It is a lab and textbook word, not a kitchen word.