The diagram shows a typical series circuit powering 3 LEDs.
This type of circuit is used when the power supply can provide enough volts (pushing power) for a few LEDs at once. For example a 12v supply could power 3 LEDs that need 3.5 volts each (3.5 + 3.5 + 3.5 = 10.5).
Also we can use any colour of the same basic type of LED in this circuit, so it's fine to mix yellow, red and green for instance.
It's also fine to have five 2.1 volt LEDs in series; any combination as long as it's less in total than the voltage of your power supply.
It is better to have your LEDs adding up to as near 12v as you can sensibly get because the resistor's job is to 'burn off' the spare power. In other words, a circuit with three LEDs will use just as much power as one with five LEDs (but only give 3/5 as much light) - the resistor will be burning off the extra and turning it into heat.
If you want to learn how to calculate resistance values for LED circuits go here.