Conjugated Bonds are alternated single and double bonds.
The proximity of the alternate pi bonds allows them to interact across the single bond axis, giving a partial double bond.
These conjugated systems absorb light and their pi bonding electrons become energized to the antibonding state. This excited state is relieved by propagating the energy down the conjugated chain, by passing an electron of fluoresced light into another conjugated pigment molecule, and finally by releasing an electron into the electrical circuit. The circuit returns an unexcited electron to the system, after its extra energy is used to do work, to fill the 'hole' left by the leaving electron.
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Picture from: Wade 4/e, Organic Chemistry, Prentice Hall 1998