Resonance Structures and De-Localization
When you draw a molecule, you are not drawing what it actually looks like. What you are really drawing is one of its resonance structures.
There are multiple ways to draw the Lewis structures for many of the chemical compounds we use, each of which is a simplification of the real molecule's structure. These are Resonance Structures.
The structure of the ACTUAL molecule is a combination of these different structures, called a Resonance Hybrid.
Different resonance structures do not mean you are dealing with a physically different molecule - it is the same molecule being drawn as a simplified form of the actual structure, because delocalized electrons are really, really, really hard to draw.
For example, Resonance Structure A might have a positive charge on the second atom, while Resonance Structure B might have the charge shown on the third atom. They are both the same molecule, and on the actual molecule the two atoms share the charge, but we draw the charge on one of the two atoms for simplicity when making Lewis Structures.
In the actual structure, the charge being spread out over many atoms is called "de-localized", because the charge is not local to any specific atom. (Think of the atoms as cities, where you are local if you live there, but you are de-local if you move around between a lot of them.)
De-localizing charges moves like charges further apart, so compounds that have resonance are more stable than compounds that do not. These are "resonance-stabilized" structures.
The stability of a specific resonance structure shows us how close it is to the actual structure of the molecule. The more stable the resonance structure, the closer it is to the actual structure. The less stable the structure, the less accurate it is when compared to the actual structure.
More stable structures: Major Contributors.
Less stable structures: Minor Contributors.