The northern promontory of Númenor, a land of rocks, cliffs and high moors. It stretched some two hundred miles from its southern borders with the Mittalmar to its North Cape. The great road from Rómenna to Andúnië passed through its southern parts, at the place known as Ondosto.
The lands farther north, toward the Cape, became rockier and more mountainous. In the far north was a region around the peak of Sorontil, where the landscape fell sheer into the sea forming tremendous cliffs. This was the abode of Eagles, and also the dwelling-place of MeneldurElentirmo, heir to Númenor's throne and a watcher of the stars, who built a great tower here to follow the movements of the heavens.
'Northlands' is the standard translation given by Tolkien. Though the for- element is very well established as 'north', ostar doesn't seem to literally mean 'lands'. It's possibly related to the stem os-, meaning 'round' or 'about', so that the Forostar would be one of five regions (along with the Andustar, the Orrostar and so on) surrounding the central highlands of the Mittalmar. Alternatively, ostar might refer to enclosures or homesteads within the region, making Forostar literally 'enclosed lands of the north', or the word might have some other meaning, yet to be discovered.