I can summarize the rules pretty easily.
You know X number of words. These are the only ones you can use to talk to other people, though you can gesture and grunt.
If you want to succeed at a task, roll a d6. If you have a skill related to the task, you need a 3 or better. If not, you need a 6. If you roll a 1 you fail hilariously badly.
If you want to hit someone with something, roll a d6. You succeed on a 5. This can be modified slightly depending on what you're hitting and your character's abilities.
You have a "class" which is a sort of caveman archetype that gives you a bonus on certain things. An "eloquent" caveman knows extra words. A "learned" one has more skills. A "tough" one takes less damage. Etc.
That's basically it. If you play long enough to gain experience, you gain it in the form of new words.