what do u mean by downhill movement of electrons

 The movement of electrons from excited PSII  to electron acceptor then to electron transport system during photosynthetic reactions  is downhill, in terms of an oxidation-reduction or redox potential scale. Movement of eletrons from lower redox potential to greater redox potential is termed as down hill movement of electrons.

The electrons are not used up as they pass through the electron transport chain, but are passed on to the pigments of photosystem PSI and then to NADP+ 

As the electrons move downhill in the electron transport chain, they lose potential energy and ATP molecules are formed .