The Brazilian jiu-jitsu ranking system is much stricter than those of most martial arts.
Depending on training intensity, talent etc. it can take up to 10 years or more to gain a black belt.

Originally, reaching the next color was not contingent on practice time measured in years or hours and certainly not depending on the attendance of paid seminars.

Every trainee would rather be evaluated based on his progress in training, his commitment as well as his mental strength and development.
On his blog, Nicolas Gregoriades wrote down the references of the different belt colors that are most important to him.
Personally, I like his deep understanding for BJJ so I’m going to recite those here in my own shortened version and I’m going to add some explanations of my own understanding in parts where I think it’s needed.

White Belt

As a white belt, it should be your goal to set up a frame of reference for the future.

Nicolas says, in this phase you learn what you not yet know. It’s about building a basis, any further knowledge is built on. By the end of your first sparring session you will know exactly what he means.

This is also the moment when a certain resignation and demotivation sets in. Unfortunately, many of the initially interested people stop their journey here before it even started.
As a white belt it should be your goal to understand the basic principles, to train your body as well as your motivity and detect new aspects of both. The experience that strength is only of secondary importance also takes place in this stage of learning.
The so-called flow diagrams are a very good tool to get an overall view and to conceptualize your training. A flow diagram can also help you to learn the basic terms for positions and techniques much easier.

Learning how to relax is another absolute basic ability for a white belt to accomplish. Relaxing is the only possibility to seize control of your ego blazing up.

Your motto should be “fun and experience” instead of “winning or losing”!

Blue Belt

The basic references of a blue belt are a solid defense from all BJJ basic positions. Every blue belt should have at least two solid escapes from mount, side control and back mount in their repertoire.

Furthermore, surviving the partner’s guard as well as opening and passing the guard should be solidified and perfected.

Purple Belt

The purple belt is the phase where movement and dynamic are integrated into your own game. Using leverage, momentum and your own weight to sweep or dominate your partner will become an essential element of your training.
Learning combined offence sequences and a very solid closed & open guard is another important aspect of a purple belt. To eradicate existing weaknesses should top off the phase of the purple belt.

Brown Belt

Although you’re not yet a master of the art, there are only very few positions or situations that you’re not familiar with as a carrier of the brown belt. In this phase you will solidify and perfect your own fighting style, rhythm etc.
The motion sequences of the techniques are nearly perfect and over time the repertoire of techniques has grown impressively.

Black Belt

A black belt is a master on and off the mat. His movements are effective, his techniques perfected and his manner should be humble and polite.

Robson Moura describes jiu-jitsu as an ocean, that has a beginning but no end. Hence, the ‘jiu-jitsu journey’ is not over even for a black belt.

Red Belt

The jiu-jitsu red belt is reserved for the great and special personalities, who have advocated for jiu-jitsu over decades, who have lived jiu-jitsu on and off the mat and who despite all ups and downs have stayed true to jiu-jitsu and thus, true to themselves.

For me, it’s the very definition of true heroes!
One of those very heroes, Helio Gracie, once made the – in my opinion – most accurate statement about the belt’s color and importance:
“The belt is just good for holding your pants.
It is too thin to protect your ass from being kicked!”
I don’t think this needs any elaboration!