Can somebody please explain "stocks" in laymen terms?

my 11-year-old and i just finished watching a censored version of “Trading Places” with eddie murphy and dan ackroyd. i tried to explain that next to last scene in the trading pit as buyers and sellers contemplated “orange juice” futures, but i don’t get it myself actually.

how can people in a pit change the direction of an economy and still profit for themselves?

can somebody please explain “stocks?”