Chlorophyll absorbs light, which then causes it to lose an electron. The electron is taken by the primary electron acceptor, which distributes it into a complex of proteins that pass it from molecule to molecule like a game of hot-potato. This is called the electron transport chain.
The point of the Light reactions is to make large quantities of NADPH and ATP. These molecules store chemical energy that is used to power the dark reactions. The cell needs this energy because making sugar is hard work.