It just means you hit the hardest (or extremely close to) the max damage you could have done for that hit. Just like regular damage you can hit for example (random numbers for explanation) 500k-1m, a crit max possible damage would be 10m-50m (respectively). A red crit means you hit really close to that number. It is possible to have a yellow 40m crit but a 15m red crit. It is like a little "Hey look how much damage you just did!" kind of thing.