<I understand that I can not connect an LED directly to a battery because it will draw too much current.

that's not quite true - as it depends on a lot of factors.

the issue with leds is that 1) once they start to conduct, a little bit increase in voltage will create tremendous increase in current. with the right combination, that can mean damages; 2) as the leds heat up, their forward voltage drop declines, which causes the current through the leds to go up. that in turn causes the power dissipation on the leds to go up, and the leds heat up. that leds to a vicious cycle.

So one way to avoid that is to introduce negative feedback so that when the current in the leds goes up, the voltage across the leds goes down.

many ways to do that. resistors, sensors, active controls, etc.

I understand that I can not connect an LED directly to a battery because it will draw too much current

That's not quite true - as it depends on a lot of factors.

The issue with leds is that:

1. once they start to conduct, a little bit increase in voltage will create tremendous increase in current. with the right combination, that can mean damages.

2. As the leds heat up, their forward voltage drop declines, which causes the current through the leds to go up. that in turn causes the power dissipation on the leds to go up, and the leds heat up, that leds to a vicious cycle.

So one way to avoid that is to introduce negative feedback so that when the current in the leds goes up, the voltage across the leds goes down. Many ways to do that. resistors, sensors, active controls, etc.