this is the middle jamraat being pelted with stones.
here's a little bit of history behind the hajj ritual of pelting the 3 jamraat (devils/satan) with stones.
abraham, or ibrahim as he is known in islam, had a son called ishmael who he loved dearly. allah decided to test him, to see who he loved the most, and sent him a command that he should sacrifice his son for allah.
this pained ibrahim but nevertheless he set about doing what his lord had told him to do. on his way to the place of sacrifice, satan appeared before him three times in three places, each time trying to persuade ibrahim to do something other than what allah had said, each time being more and more persuasive. these places are represented by the 3 pillars.
eventually, ibrahim got to the place of sacrifice and was getting ready to sacrifice his son, when allah explained to ibrahim that it was a test and that he'd passed it. allah sent a ram, which ibrahim was told to sacrifice in place of his son. this is also why we celebrate eid-ul-adha with a sacrifice.
in this way, you'll realise that a lot of hajj rituals are to do with prophet ibrahim (peace & blessings be upon him).
