resonance
Suppose you try to describe a mule by drawing a horse and then a donkey — neither sketch is right, yet the real animal is a single blend of both. Some molecules are like that: no single Lewis structure captures them, and the truth is a blend of several reasonable drawings. That situation is called resonance.
Precisely, resonance is the description of a molecule whose real electron arrangement cannot be shown by one Lewis structure, so it is represented as a combination (a hybrid) of two or more valid structures that differ only in where some electrons sit. The actual molecule is not flickering between these forms — it is a single, fixed structure that is the average of all of them, lower in energy than any one would be.
Resonance matters because it correctly predicts that certain bonds are all identical and intermediate in length and strength, and it explains the extra stability of molecules like benzene. The crucial caveat: the contributing structures are not real and the molecule does not switch among them. They are a limitation of the Lewis dot language, which forces electrons into fixed spots when they are actually spread out (delocalized).
The carbonate ion (CO₃²⁻) can be drawn three ways, each putting the double bond on a different oxygen. In reality all three C–O bonds are identical, each with a bond order of about 1.33 — the molecule is the single average of the three drawings, not any one of them.
Carbonate's three equal bonds are the average of three resonance drawings.
Resonance is not the same as equilibrium. In an equilibrium, real molecules genuinely interconvert back and forth. In resonance, nothing converts — there is only one molecule, and the multiple structures are just imperfect snapshots of its single, delocalized electron arrangement.