Reactive: A term used to describe something that readily undergoes chemical changes. This would be like a shaky Lego tower ready to collapse at the slightest touch.
Inert: Describes substances that are not chemically reactive. Imagine a Lego structure glued together - it's not changing no matter what!
Equilibrium:The state in which both reactants and products are present in concentrations which have no further tendency to change with time. Like having just the right amount of Legos to keep your structure balanced and steady.