The choke is a constriction at the end of the barrel. It is designed to keep your shot pattern tighter for a greater distance. A modified choke is 20 thousandths constriction. Nominal 12 gauge bore diameter is 0.729" so a a modified choke has a diameter of 0.709" at the muzzle.
You do have to shoot rifled slugs in a choked barrel because the slug needs to be able to 'squash' a bit going through the choke. That's what the rifling on rifled slugs is really for, it doesn't spin the slug like a propeller.
For smoothbore shotgun slugs, I consider 50, maybe 75 yards, about maximum effective range. At that range, pretty much any slug will give similar results. Buy a few boxes and see what your gun likes. I would try Brenneke KO, Remington Slugger and Winchester Super-X. All are economical and reliable.