Do not confuse the double-headed arrows that denote resonance with the two single-headed arrows that denote equilibrium. The molecules on the ends of equilibrium arrows are distinct molecules that can undergo a chemical transformation to become the other. Resonance is not an equilibrium in that the resonance structures don't actually exist on their own. A resonance hybrid is not the average of resonance contributors that fluctuate from one to the other.
In the next chapter we will discuss isomers, molecules that have the same molecular formula but are different in some way. It is important to note that resonance structures are not isomers of one another, simply because they don't actually exist. Another common question is of the following nature: The molecule propene can be written in two separate orientations, but both represent the same molecule; in other words, the structures are identical. Now consider the resonance structures of the allyl cation. Aren't these identical too? And if so how can we consider them distinct resonance structures? Once again, the answer is that the resonance forms are only representations of the true allyl cation; the resonance forms themselves don't exist. The positioning of electrons on resonance structures is all-important. Seemingly similar resonance forms are often necessary in order convey the true nature of some molecule.
In our earlier definition of resonance we said that the resonance hybrid is a weighted average of its resonance forms. In fact, not all resonance structures contribute equally. As an extension of the color analogy, imagine that we try to describe a very dark shade of gray as a hybrid of white and black. Clearly, dark gray resembles black much more than it resembles white. Such dominant resonance forms are said to be major resonance structures, whereas the less dominant forms are minor resonance structures. For instance, both resonance forms of the allyl cation contribute equally (hence both are major). However, the acetaldehyde enolate is dominated by the major form on the left: There is a simple rule for determining which resonance forms are major: More stable resonance structures contribute more greatly to the resonance hybrid. Since the two resonance forms for the allyl cation are equally stable, they are also equal contributors.
The ability to roughly assess the stability of organic molecules is an important skill, and one that you will acquire as you increase your intuition of organic chemistry. For the enolate example, we can use a simple electronegativity argument to justify why the left form is more stable. Because oxygen is more electronegative than carbon, the negative charge is stabilized more by residing on oxygen. Thus, in the resonance hybrid, we would expect oxygen to bear a larger partial negative charge than carbon. We would also expect the C-C bond to have greater double-bond character than the C-O bond.
We have seen that resonance structures can be used to describe molecules whose electrons are not fixed on specific atoms or bonds but are spread out over several atoms or bonds. This phenomenon is called electron delocalization. Delocalization is an energetically favorable process: by distributing charge over a greater volume, the net energy of the molecule is lowered. The result is that a resonance hybrid is more stable than any of its contributing resonance structures because of delocalization in the resonance hybrid. Hence, resonance hybrids are said to be resonance stabilized. An important guideline in organic chemistry is that a molecule becomes more stable whenever more resonance structures can be drawn for it. Additional resonance structures, even those relatively high in energy, always stabilize; they never destabilize. Of course, the more stable the additional resonance form is, the more stabilization the resonance hybrid receives.