11-cis-retinal has several conjugated ("neighboring") double bonds, which makes it easier to absorb photons in the visible range. The double bonds can share their electrons, which means there's a longer system for absorbing longer-wavelength (in this case visible) photons efficiently.

Once you add all this energy, the 11-cis double bond gets flipped into the trans conformation (the process is called photoisomerization). That removes the bend in the retinal molecule, which pushes on the protein, rhodopsin, around it to change its shape. With its new shape, the rhodopsin can bind to and activate a trimeric (three subunits) G protein. The trimer falls apart, and one part called G_alpha goes on to bind and activate a phosphodiesterase enzyme that destroys cyclic GMP. This affects ion channels that are opened by cyclic GMP.