Expected Value (EV) is how much you would expect to make, on average, if you repeated an experiment (or gamble) a large number of times.
If you pay $1 for a one in five chance of winning $2, then the expected value is $0.40 and you might decline the offer.
A gamble with an expected value of more than $1 per dollar is only worth taking if you can afford to take the loss. Russian roulette is an example of a game you shouldn't play regardless of the EV, because you can't afford to lose.
Lottery generally has an expected value of less than the face value of the tickets. But in a jackpot scenario the EV is higher, and can be higher than the face value of the ticket.