Electrons belonging to certain molecules are not attached to a particular atom or bond in that molecule. These electrons are said to be "delocalized" because they do not have a specific location (are not localized); they cannot be drawn in a simple Lewis structure. Rather, they exist in orbitals that include several atoms and/or bonds. You can imagine these orbitals as clouds surrounding parts of the molecule. Delocalization gives molecules resonance stability, stronger acidiy and based on the resonance stability, we can determine the range of absorbtion of ultraviolet and visible light. of a molecule in the light spectrum. The actual structure with delocalized electrons is called a resonance hybrid.  According the valence-bond theory (resonance hybrids are structures whose forms are a mix of several representable forms. It is a mixture. This occurs because double or triple bonds can form between atoms). One of the many structures, with localized electrons, that are used to represent the hydrid molecule  is called a resonance contributor. Electrons become delocalized in order to stabilize a structure. For example the benzene molecule, C6H6, delocalized electrons to stabilize its structure rather than having alternating double and single bonds, and is frequently drawn as a circle inside a hexagon to represent the shared electrons.  Also,  acetic acid, CH3COOH, has delocalized electrons that stabilize its conjugate base and thus make it acidic.