When amino acids combine they do so via the formation of an amide
bond between the alpha-carboxyl of one amino acid and the
alpha-amine of the other. This amide bond is termed a
peptide bond.
(1).
(2). (colored in blue)
The peptide bond has two resonance forms which makes the bond have
double bond character. Due to this double bond character rotation
about the peptide bond is restricted. This in turn restricts the
possible motions of the peptide backbone.
In addition to the restriction of rotation about the peptide bond,
its double bond character also places six atoms of the peptide
group in the same plane. This plane is called the peptide
plane.
(3).
(4).