alkene stability
Not all double bonds are equally content. Alkene stability is the observation that a C=C carrying more carbon groups (more substituents) is lower in energy — more stable — than one carrying fewer. So a tetrasubstituted alkene (four carbon groups on the C=C) is the calmest, and a monosubstituted terminal alkene the most restless, with di- and trisubstituted ones in between. Among cis/trans pairs, the trans is usually a touch more stable than the cis because its bulky groups sit farther apart.
How do we know? By measuring heats of hydrogenation: when you add hydrogen across different alkenes that all collapse to the same alkane, the one that releases the least heat must have started out lowest in energy. The reason for the trend is partly hyperconjugation — neighbouring C-H bonds donate a little electron density into the pi system, and more alkyl groups means more such stabilizing donations — and partly that an sp2 carbon prefers to bear a more-substituted, electron-richer partner.
This little ladder of stabilities quietly governs a lot of outcomes. It is why Zaitsev's rule predicts the more-substituted alkene as the major elimination product, why some isomers form in preference to others, and why the relative energies of carbocations (the more-substituted, the more stable) echo the same logic. Whenever a reaction can give a choice of double-bond positions, alkene stability is usually whispering the answer.
Of the butene isomers, 2-methylpropene and trans-2-butene (both disubstituted) are more stable than 1-butene (monosubstituted), as shown by their smaller heats of hydrogenation when all three are hydrogenated to butane.
More alkyl groups on the C=C means lower energy and a smaller heat of hydrogenation.
The cis-is-less-stable rule comes from steric strain between groups crowded on the same side; it can flip when the groups are unusual (for example, certain cis fluorinated alkenes are the more stable isomer).