Hybrids are (Lewis) abnormal
Resonance theory is applied to molecules with abnormal electron patterns (bonds and charges). If a molecule's electron pattern cannot be described by a single Lewis structure, the molecule is called a resonance hybrid and resonance theory tells us how to think about the molecule.
What is abnormal?
We have to be careful how we think about "abnormal." It does not mean unusual.
CH3+ is a highly reactive, very short-lived ion. It is hard to make, difficult to detect, and unusual, but it is not abnormal because its properties are consistent with its Lewis structure.
On the other hand, ozone, O3, is produced worldwide on a daily basis by sunlight, lightning, and auto engines, and is abnormal. To see this, we must compare its structure and properties with its Lewis structure(s).
First, ozone's properties: the two OO bond distances are identical (128 pm), and so are the left and right-hand sides of its electron density cloud (both of the following pictures show left-right symmetry).
Ozone models. above: 0.4 isodensity surface. below: potential map (default).
Now we ask whether any reasonable Lewis structures for ozone are consistent with these properties. Formulas I and II look good because every atom possesses a Lewis octet, but these drawings do not have left-right symmetry. Formula III is symmetric, but it shows OO single bonds, which are much longer than the OO bonds in ozone. III also makes every O neutral, while the potential map shows the middle O is electron-poor.
Since none of these Lewis structures describe ozone adequately, we must accept the fact that ozone is abnormal and call it a resonance hybrid.
Is a little abnormal the same as a lot abnormal?
No. The distinction between normal and abnormal is murky in the molecular world, just as it is in the human world. A molecule that seems only a little abnormal, that is, its structure / properties deviate only a little from what one predicts using its best Lewis structure, is usually regarded as normal.
Consider the best Lewis structures for H-O-O-H, CH3-O-O-H, and CH3-O-O-CH3. These structures show identical single OO bonds, but the actual OO bond distances vary (145-148 pm). Does this variation mean one of the molecules is abnormal (a resonance hybrid)? No, because the variation, although real, is very small. (For comparison, ozone's OO distance is 128 pm.)