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Some resonance structures are more stable than others. The stability is determined by the formal charges, so structures with the lowest formal charges would be more stable. The most stable structure is the major contributor.
Just to add, elements want to have the same amount of electrons belonging to them as they have valence electrons, so the more atoms that can fulfill this requirement the better the structure. Some atoms are more likely to accept a negative formal charge due to their electronegativity, and some atoms do not want positive charges because they are very electronegative.
To add another way to understand the concept, resonance structures provide different areas for electrons to go in order for the molecule to become more stable! Because electrons are delocalized/can move freely, the molecule has lower energy and thus is more stable.
When a molecule has different resonance structures, the only thing that changes is the arrangement of electrons (structure of atoms, # of lone pairs, and overall formal charge remains the same). However, some resonance structures are more stable because the formal charges of the individual atoms are 0 or closer to 0 than the other resonance structures.
Certain resonance structures have a more stable formal charge since they are closer to 0. Structures with the least amount of formal charge tend to be the most stable, and thus, they are the resonance structures that contribute the most to the molecular structure.
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