It means to hang out and chill and do some sort of activity that does not require too much effort. Usually used in reference to doing drugs or taking mind/body altering substances, but does not have to be used in that content necessarily.
Mike: Hey John, come tone with us for a bit. We are just watching some South Park.
Bryan: Hey man wanna come tone down a bit with me and Paul? (In reference to marijuana)
Joe: Nah man I can't be tonin when I got an 8 am test tomorrow.