It's basically a (rough) description of the relative wind speed on the site compared to the local average. If a site is sheltered, it will typically experience: lower wind speeds,
potentially warmer mean temperatures, and (in temperate zones) a lower risk of frost (although some sites may actually have an increase due to catabatic airflow: "frost hollows")
Depends on the context. Examples can be: In depressions instead of lonely on the plain, inside of the forest instead of at the edge, along a river bank or lakeside instead of on dry ground.
Or, in short: exposed to wind & weather instead of sheltered from the rigour.
Also depends on the tree. Some pioneer trees can grow individually in adverse ...
The Cimarron River, a salt river, extends 698 miles (1,123 km)
across New Mexico, Oklahoma, Colorado, and Kansas. The headwaters flow
from Johnson Mesa west of Folsom in northeastern New Mexico. Much of
the river's length lies in Oklahoma, where it either borders or passes
through eleven counties. There are no major cities along its route.