Eventually, 24 will be selected to make up six crews of four, which Mars One says they hope to launch to the Red Planet every two years from 2024, with the aim of starting a colony there.
What does the sentence mean in red?
It means that the 24 selected people will be divided in groups of 4. Each group will be a crew on the Mars One mission (to visit Mars).
Please be aware that I'm neither a native English speaker nor a teacher.