pot odds are the ratio of the total amount in the pot to the ammount you have to call.
For example if the pot is $5 and villain bets $5 you have to call $5 to win $10 ($5 + $5).
This gives you pot odds of 2:1 (10:5).
Being able to calculate pot odds is useful when it comes to deciding whether to continue in a hand.
A simple scenario may be:
You hold AdKd on the button.
Board reads 10d 7d 5c 2h.
Current pot is $5 and villain bets $2.50.
This gives you pot odds of 3:1 on your call ($7.50:$2.50)
In this spot, let's say villain had QQ and you knew his hand, you have roughly a 25% chance to win the hand (25%
equity) so calling here would be a neutral EV move (EV =
expected value). Or in other words the pot odds are 3:1 and you will win 1 ($7.50) for every 3 you lose (3 x $2.50) so the call is break even. The money you win every time you make your hand would equate to the same you lose every time you don't, over a large enough sample.
However if you feel that you will get another bet out of villain most of time that you make your hand this move becomes + EV as you will win more when you hit than you will lose when you don't. This is called your implied odds (as you do not yet know they are concrete, they are only implied by the action taken place this far) and can be useful in decision making in various scenarios.
An common example of when implied odds would be used to influence decision making would be when holding a pocket pair pre-flop. You may only be getting pot odds of 3/2 against a very tight villain opening range, where your equity could be only 20%, but if you knew you would hit a set 1 in 10 times and win the rest of the villains 100bb stack every time you did you have the implied odds to call, even though not the pot odds. The money you could win when you hit (1 x 100bb), which you will do roughly 1/10x far out ways the amount you will lose when you miss ( 3bb x 9 ).
Implied odds are actually more complicated than the above scenario but I tried to keep it as simple as possible for explanatory purposes.
I hope this made sense.
The simplest way is to work out your clean outs/equity against your opponents perceived range as a rough percentage or fraction, then do the same for your pot odds (how many times the amount you have to call goes into the total pot). If they are the same or better a call is good, if you are over 50% against your opponents perceived range to win the hand you can raise for value.
** note: I know the percentages above are not exact, just give a rough idea to keep it simple.