FLL is an international competition that helps to develop real life skills such as presenting, critical thinking, spontaneous reactions, and innovation. You have to work together as a team of 2-10, and you work together to program, create slideshows, act in and write skits, and more. There are four main categories of FLL that each make up 25% of your score.
The first one is your robot run. For this, you get a mat with all sorts of pictures. You also get Legos to build obstacles/missions that a robot you design and create out of Legos as well. These missions have different point values, depending on their level of difficulty. You may not use anything but Legos for said robot, keep in mind that there are Lego rubber bands and a lot of other cool things. You may not touch your robot or anything outside of a corner that is painted white and it's end is clearly marked with a black line. Once your robot comes completely past said black line though, you are free to touch it. If you were were to touch the robot when it was outside off base, then this would result in a penalty, meaning that 3 points would be deducted from your final score, for each penalty that was earned.
One example of these missions is, say, you have a Lego somewhere on the mat and you have to move it within the black line of your base, then your robot would go, pick it up/trap it, and then return to base with this Lego.
You must complete as many of these missions as possible in 2:30 minutes.
The second part of FLL is your robot design presentation. Here, you have to explain to volunteer judges how your robot is sophisticated and unique. You can explain how you used different types of sensors, motors, and modular attachments, how your robot is unique and sturdy, how your robot doesn't use more pieces than it needs to, etc. The more varied and ample these are in your robot, the more points you will get from the judges. You can also print out your program and show the judges these papers, and if you explain it to them, they will understand and be able to follow along more easily, allowing to explain more deeply what you are doing.
The third part of FLL is your Innovation Project. Each year, there is a theme, and you have to come up with a real-world problem that is related to this theme. There are also missions, and they are related to the theme that year as well. For this, you have to come up with a problem, research it, and find an innovative solution that is achievable, and could possibly be implemented into a real world application. You also want to make sure that you present your project in an exciting way, as you will get more points from the judges if you are able to entertain them.
The last part of FLL is core values. Core values assesses how well your team worked together, and how you were with other teams. You would get points if you meet another team and share your project, called your outreach, or if you displayed coopertition, which is a combination of cooperation and competition. The judges give you an objective, normally you have to build something related to that year's theme with certain resources, and the judges will score you mainly based on your teamwork but also your ability to complete the objective.
For people just starting FLL, it is a great competitive activity in which you can develop researching and simple block coding skills, and whether you are in 4th grade or you are 14, I would definitely recommend it as a fun way to socialize and learn.