A red bar above the player's model indicates the successful cancel of an ability.
Interrupt, or Ability Canceling, is a mechanic in Battlerite that allows the player to cancel their ability during the casting animation or during abilities that are channeled, such as Shifu's 
Whirlwind, Ruh Kaan's 
Reaping Scythe or Sirius' 
Astral Beam. This allows a player to bait out an opponent's abilities if they try to preemptively act before the ability is fired.
The default keybind for Interrupt is set to "C", but can be changed according to the player's preference.