The phrase "to pit the cherries" means to remove the pit or seed from cherries. It is the process of taking out the hard center from the fruit. Full definition
You could use fresh pitted cherries when they are in season though. (connoisseurusveg.com)
Pit the cherries ~ it's not the easiest thing, mine ended up in thirds and quarters but it doesn't needs be neat! (thewholefooddiary.com)
Prepare the cherry jam by cooking pitted cherries is two table spoons of water until they are soft and break easily when pressed with a spoon. (veggiesouls.com)