In a hand of two players the effective stack is the amount of big blinds of the player with the fewest of them. Making the rest of the blinds of the other player ineffective since can’t be betted.
This is a very important concept to understand because the effective stacks will dictate which hands you should play and how you should play the hand.
If the effective stacks are shallow in a tournament and the player with a short stack enters the pot with a raise he is very likely to be doing this with a strong range and you should only call or raise with a very strong hand that can beat their opening range.