Difficulty is estimated by the creator of the project. This estimation can can be modified by the administrators of the site depending on the feedback of users and to adapt it with the other projects published.

very easy, even for young children (7-8 years old)
average difficulty. For young children, the help of an adult is necessary.
quite difficult. You must be handy even if you are not a specialist
difficult. The help of an adult in needed. You must be handy to succeed.