The Chargin' Targe is a shield for the Demoman, often used in combination with a sword to make a "DemoKnight". It starts with an initial charge, and upon right-clicking your mouse, the demoman will zoom forward, increasing their damage with the sword/pan/bottle (by making attacks crits / minicrits). You will often see them slicing the heads off of some people with plenty of blood once they stop moving. You need to be wearing the shield to use it. A charge bar will be in the bottom right corner.
The Splendid Screen is the other Demoman shield, which does more damage than the Chargin' Targe, but has fewer bonuses.
Recently, they added a third shield called The Tide Turner, which has the unique ability to fully recharge the charge meter if the player makes a kill on the charge.
Tactically, from my experience, it's a fun thing to do by charging into a crowd and chopping off a few limbs. Each takes 12 seconds for a new charge up.
Equipping the Persian Persuader melee weapon will grant the user 100% increase in recharge rate.