Effective odds. If you gonna count on the odds after flop it's uncertain how much it would cost to see
the remaining cards turn and river.
We suppose it's $10 in the pot after flop and the only opponent left has just made a $5 bet
and your pot odds are 3-1.
It's two cards left for you to make your hand and you must guess how much it will cost
you to see the next card.
If you only have one opponent left and it's fixed limit, then it's easy to know. Suppose your opponent
bet $5 on turn and you call. Then you have to pay $10 ($5 on flop and $5 on turn) to win $20.
If you're lucky maybe your opponent bet on river and you make your hand and you can raise.
An example:
You have 48
and flop is 7A6
your chance of hitting flush or nut straight with 12 outs is 12/47+
35/47 *12/46 = 0.45, then it's good to call or raise.