Tabard represents a faction in WOW, wearing it means that you made a corresponding contribution to this faction. There are hundreds of factions added from the earliest patch to the current Legion Pre-Launch. Different kinds of tabards in these factions maybe tell you a special history.
The "Tabard of the Scarlet Crusade" drops from The Ravager Armsmaster Harlan in Scarlet Halls and drops with a chance of 0.77%. Originally, Scarlet Crusade is established for fighting against the scourge with good intentions, but because of its crazy and blind, scarlet crusade has made numerous abuses under the name of justice.  
The "Of Love And Family" quest and Scarlet Crusade set are well known in the 60s.
Arise, my champion!
At your side, my lady! 
Every player must be very familiar with these two slogans. 
You can buy two transmog sets(Plate & Mail Armor) from crusade quartermaster after wearing Tabard of the Scarlet Crusade when Darkmoon Faire event is coming. It is a memorial to the past scarlet crusade set.