Older style (fiberglass) bedding was poured/fitted only to the upper side of stock where action, recoil lug, and barrel contacted the stock. Barrel itself typically was glassed/bedded for the first inch or two forward of receiver and then floated the rest of the way OR could have a pressure point at front of fore-end.
With pillar bedding the same thing occurs but some type of fiberglass/metal is inserted between this top bedding layer and the bottom of stock, surrounding each of the two action bolts. These vertical "pillars" prevents the stock from being crushed vertically by someone overtightening action bolts. Thus being more consistent and usually delivering better accuracy.
Good shooting and be safe.
LB
