Can someone explain what's the function of Galio's W and when to use it? I just don't get it.

https://clips.twitch.tv/HorribleWrongJaguarPermaSmug

Its a defensive steroid with hard cc attached to its 2nd cast, you basically use it to help you tank/zone enemy backliners and and taunt enemies within it, ideally you do both with 1 cast

Galio's W is does a couple different things 1 being he gets a passive magic shield 2 you activate it by holding it and the releasing 3 when its charging up you get this circle around you that gets bigger as you hold it and any enemies inside the circle are taunted and are forced to auto attack you 4 while your W is charging up you get a little bit of damage reduction

Thank you all

You cast it on yourself or an ally when an enemy is about to burst them to offset the damage done with healing???

Passively, it helps with his "anti-mage" role. Actively, it gives him some AoE CC (and somewhat references his old ult of a taunt.)

E+Auto+Q+W until CC wears out+Auto gives you 2 passive hits later and tons of damage since the W keeps them in the Q and your Aftershock. Also it is one of the best CC tools in the game since it effectively has a range of like 600+ with Flash.