Imagine you're betting on the roll of a fair 6-sided die.
Let's say you are getting odds
of 8-1 on the roll being a 3. There will be times that a 3 doesn't appear for dozens of rolls. Of course, this doesn't mean that you should change your bet if you can only get 5-1 on rolls of 1, 2, 4, 5, or 6, because, as long as the die is fair, continuing to bet on 3 is clearly going to be the only profitable option in the long run. If you encounter one of those 30-roll stretches the results are telling you to pass up a hugely +EV opportunity, which clearly you should not be doing.
With regards to EV, let's say you place $1 on the aforementioned die rolling 3 at 8-1. 1/6th of the time you win $8 (net), 5/6ths of the time you lose $1. Your EV is (1/6)*8 less (5/6)*1, or 50 cents, in other words in the long run you can expect to gain 50 cents per $1 bet you put on a 3 to roll.
The same is true of poker, EV is basically what you stand to win multiplied by the probability of you winning less what you stand to lose multiplied by the chance of you losing.