First off, it's all based on whoever has the shortest stack. If you have $50 and he has $10 you can only win $10. Or if you have $10 and he has $50 you can only win $10.
Next it's figuring out your chances of hitting a winning hand.
Pre-flop, you could be looking to set mine with a small pair. You only flop the set about 1 in 8 times of the times so you need to be able to get paid off at least about 12x what you bet. Yes, this is greater than 8x, because a lot of times you'll hit but his A/K (or whatever) won't connect so you only win a small pot.
So basically if you have 3,000 chips and 5/5 in the hole, and someone with 1,000 chips opens to 120 you fold. You won't get your set enough times to make it worth it.
But if you have 3,000 chips and 5/5 in the hole, and someone with 6,000 chips opens to 120 you call. It's even better if someone before you calls that raise as your chances of getting paid off if you hit the set increase and the potential pot size is much greater.
With an open-ended straight, you have 8 cards that will probably give you the winning hand. You said you knew the 2 & 4 rule. So with 8 outs, you will hit your card about 32% (one third) of the time. So the idea is to only call bets that are small enough to allow you to lose those 2/3 of the times and still make a profit.
Implied odds are the extra chips you think you can win if you actually hit your hand.
A simple example of this would be if you both had 2,000 chips and the pot was 300 on the turn. He bets 150, making the pot 450. So you you get 3:1 odds but you only hit your flush 20% of the time, which are bad odds. But if you do hit, you can raise your sure-winner on the river to inflate the pot even more. So that 150 you are betting is giving you a chance to make another raise on the river so it is
implied that you will win even more money if you hit.
1/5th of the time you call 150, the pot is 450, you miss the suit, he raises to 350, you fold. $-150
1/5th of the time you call 150, the pot is 450, you miss the suit, he raises to 350, you fold. $-150
1/5th of the time you call 150, the pot is 450, you miss the suit, he raises to 350, you fold. $-150
1/5th of the time you call 150, the pot is 450, you miss the suit, he raises to 350, you fold. $-150
1/5th of the time you call 150, the pot is 450, you hit the suit, he raises to 350. The pot is $800 and you raise him to $900 and he calls the 500 more into the 1,700 pot (making it a 2,200 pot). 4/5 of the time you lose 150 for a 600 loss. 1/5 of the time you hit for an additional $800 on the river.
...and if the guy is willing to call a bigger bet on the river, even better.
What is implied is that if you can overcome the wrong odds to see the next card, you will have the chance to inflate the post size to make it worth it.
Oh, and it's not just about calling. You can bet in these situations. You open, they fold an inferior hand (fold
equity). Or you open, they call, and you rely on your implied odds.
