1) When wearing any shield you defend with your weapons skill against melee at +1 if you spend a supplemental action to use it.
2) Once per exchange, you can interpose a shield to absorb some of the energy of a melee blow that lands; reduce any extra stress from the attacker's high success roll by the shield rating. You still take normal stress from the weapons rating.
Buckler
Shield Rating 1
Medium Shield
Shield Rating 2. You can use the shield as partial cover against ranged attacks (guns, thrown, bows and the like). You can interpose against both meleee and ranged attacks and use the defend option as a supplemental action against a single ranged atacker. A medium shield slows you down a bit and you can't use both hands to effectively use larger weapons; you get -1 to attacks with weapons rating 2 and -2 with weapon rating 3.
Tower/Riot Shield
Shield Rating 3. You can use the shield as full cover against melee and ranged attacks; as a normal action, you interpose it against all attacks from one direction. However, a tower shield is too unwieldy to move quickly; you cannot use it as a supplemental action, and you get -1 to attacks with weapons rating 2 and -2 with weapon rating 3.
So a SWAT guy with a riot shield that wears heavy bulletproof suit (armor 2) and takes cover and gets shot from the front by some thugs with handguns is fairly safe; even if they get 3 shifts of success in their roll to hit, the shield absorbs the extra stress and the armor stops weapon 2 attacks. Then comes a guy with a machinegun. The shield still absorbs up to 3 shifts of extra stress but doesn't stop the weapon 3 attacks; the armor reduces the impact but the SWAT guy still takes 1 stress.
In his turn, he attacks back with his own gun but because the shield is in the way (-2 to hit) he misses.
In the next exchange, the sneaky thug that had crept up behind him and aimed in the first exchange now shoots, tagging "In my Sights" and "From Behind" since the SWAT guy is definitely facing away from him. He rolls great and the shield does not protect against it; the SWAT guy rolls only an average in his defense. He takes a weapon 2 hit with +2 success from rolling but another +4 from aspects for a total of 8 stress. Armor reduces that to 6 stress but he is still taken out. (unnamed NPCs don't have consequences to spare)