It literally is…
Please go educate yourself…
Lag is the time taken from an action to be noticed by the servers.
You cast a spell they were alive… if the servers were processing the data like they should it should land. However they have artificially added in a delay to that processing(the definition of lag) which means that in that delay the hit landed and it comes down to which gets processed first in that batch the mob hit or the heal on whether they live or die.