Conjugated dienes are resonance stabilized.

eg: 1,3-butadiene (1)

stability: 1a >> 1b = 1c
contribution to the hybrid: 1a >> 1b = 1c

Since 1a contributes significantly more to the hybrid than 1b or 1c, the internal pi bond in 1 is considerably weaker than the terminal ones.

It is weak enough to allow free rotation, due to which 1 exists in two limiting conformations.  The two double bonds are on the same side of the single bond in the Lewis structure of one of them, which is called the s-cis conformation (s-cis: cis with respect to the single bond); they are on the opposite sides of the single bond in the Lewis structure of the other, which is called the s-trans conformation (s-trans: trans with respect to the single bond).

Some conjugated dienes are locked either in the s-cis conformation (2) or in the s-trans conformation (3).