A skill level 2 is supposed to be for an intermediate level modeler. Glue together, some small parts, painting is needed to complete the kit as shown on the box.
Skill level 3 is supposed to be more advanced featuring small detail parts that are supposed to make the kit more realistic. Some Level 3 auto kits have doors that open, better suspension detail, etc.
I don't put much stock in these ratings. If you take your time and work carefully, they all go together fairly well. I have found that some of the hardest kits to put together were skill level 1's that had seams that were almost impossible to hide! ( King Ghidora for example)