Idiots guide to LED's
The LED will have a long leg, and a short leg.
The long leg is the Positive connection.
An LED need's maybe about 1.6v to run.
To work out what value resistor you need to place between the positive connection on the power source and the long leg of the LED, you need to use Ohm's Law.
Voltage divided by the Current equals Resistance.
If you have a 3 volt supply, and you have a 30mA LED you would require a 1k Ohm resistor.
3 divided by 0.003 equals 1000
Then you have to ask yourself if you're connecting the LED in Series or Parallel?
If you're connecting then in Series, one after the other in a chain, you'll need add all the LED's together.
So a 3v supply will run 2 LED's, and a 9v supply would run about 5 LED's.
If you connect them in Parrallel, (all the positive connections together and all the negatives together) Witchcraft happens, and you can connect lots of LED's together but you have to add all the currents together ensuring they don't exceed the maximum current of your power source.
LEDs have a specified current, which you should match within a reasonable tolerance. Since D is for Diode, you also have to satisfy the forward voltage drop across the LED. The resistor gives you somewhere to shunt the remaining voltage and a means to control the current. The forward voltage and the expected current vary. There are some standard values which are often right, but you should really check the specifications for the parts you're going to be using. Alternately, have extras and know how to use a multimeter to verify your estimated values by trial and error.
Looking at it as a way to "limit the current" is in the right ballpark but not really entirely correct. It does set the current, and it is chosen such that given the supply and forward voltages the LED will receive its specified current. But its purpose is not to prevent an over-current condition and it doesn't have a way to stop it from occurring if something happens outside the ideal expected conditions.
This is pretty handy for single LEDs and this one for arrays, especially if you don't grok circuit analysis. On the other hand, you don't need much more than Ohm's Law for this, which you should really pick up ASAP if you're going to be doing electrical stuff. Note that the array wizard will give you results which are of varying stability. One resistor per LED is the most stable (1xN array), but this is not always desirable. The overall circuit including the supply and other elements will determine whether this works in any consistent or usable fashion, so you should stick to single-LED or 1xN arrays if possible.
Undercurrents is an award winning alternative news service producing videos of people taking inspiring actions. http://www.undercurrents.org