EV means
expected value and is simply an expression for, how much you are expected to win from a certain situation. If you are all-in in a pot of 10.000 chips with 30% chance of winning, then your EV is 3.000 chips. Or if you are playing a 3-handed tournament with a price pool of $15, and each player has the same amount of chips, then your EV is $5.
However what you probably mean is chip-EV and not just EV. When we play in chip-EV, we are just trying to win chips, and we dont take payjumps into account. Lets say the 3-handed tournament is winner takes all. Then we are just trying to win all the chips, and there are no tactical considerations of trying to lock up second place first. So we play in pure chip-EV, and this is also the case early in an MTT.
Lets say its a live game, we are BB with a 10BB stack, SB has us covered and move all-in. And for whatever reason he now flip over his cards and show us 33. We have AK, so its a "coinflip". If we play in chip-EV, we call, because we are allready in for 1BB, and there is an ante, so we are getting the right odds to gamble. But if we are 3 players left in a tournament, where the winner gets $10, the runner-up $5 and number three get nothing, now we are playing in ICM-mode.
ICM mean Independent Chip Model and takes into account, that additional chips are less valuable, when multible players get paid, because nobody can win more than one price. If all 3 have the same stack, each player has an EV of $5, since the total price pool is $15. However lets say, that SB jam on us, and we look down at AK again, knowing he has 33. Now we need to fold, because if we call and lose, we get nothing. And if we call at win, we are still only sure to get second place money.
So our EV only went from $5 to $8,33, while the player not taking part in the confrontation saw his EV go from $5 to $6.67 simply by watching someone else get busted. This is because, both players have now locked up $5, and according to the ICM model the chance of winning is proportional to the amount of chips. So the guy, who dubbled up, will still only win 2 out of 3 times, and therefore his extra chips are not worth $5 but only $3.33. And for that reason any player need a huge
equity edge in order to make a profitable call.
So to sum it up ICM-mode mean, we are not only focused on winning chips but also on outlasting other players. How much depends on the situation. Its a math based situation, but luckily its also somewhat intuitive. If 7 players are left on a final table, and we are second in chips with 39BB, then its kind of obvious, we dont want to risk all our chips against the chip leader, if 3 other players are sitting on less than 5BB each. Instead we want those guys to bust and lock up some payjumps, before we take on the chip leader.