What does a bearded dragon semi-arboreal mean?

Well, in this case "semi" means partly or sometimes, (technically it means half) and "arboreal" means tree dwelling. So a semi-arboreal bearded dragon is one that prefers to spend some portion of it time up in a tree rather than on the ground.

It is commonly held that juvenile bearded dragons are often semi-arboreal, while adults tend to be ground dwellers.