ok i'll give an example you hold A9 diamonds, 2 diamonds on the flop you're up to the turn at this point and you'll need to hit a diamond on the river to win this hand, the pot is $70, villain bets $20, you have to call $20 to win $90 giving us 4.5:1.
So we have roughly 20% equity meaning we'll hit our hand 1in5 times, so 4 times we'll miss and we will lose $20x4 which is $80 right? But the one time we hit we'll win $90 therefore this particular scenario has a positive expectation (+ev) i tried to explain this as simply as i could, people will most likely be able to explain it better than me but hope i helped a little bit