system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What are the differences between lumen, lux and candela and why do i need to know?

We were talking about it in school and i understood nothing.
gpt
Lumens are how much light is emitted by a light source. Lux is how bright  a surface is (lux = lumens/m^(2)). Candela is how far you can be from a surface in order to light it up.

I shall try to explain this better. Take for example a 100mW green laser pointer. It emits about 10 lumens. A candle also emits about 10 lumens. This means that if you aimed both of them at a white ceiling, your room would be lit up about the same amount. 

The laser, however has a much tighter beam (about 2.1×10^-6 steradians vs about 10 for the candle). This means that the candle has about 1 candela while the laser has about 5 million. This means that the laser would be as bright from 5 million metres away as the candle would be from 1.

To explain lux, imagine you're in a dark room and you're trying to read a book. If the pages aren't bright enough, you can't read very well. It doesn't matter how much total light you have (gigantic pages don't help you read if the text is the same size) but it does matter how much light you have per unit area. The minimum illuminance needed to read is about 2-3 lux.

Note: The numbers in this comment may be out by a factor of 2 or so.