Fells to me would be rugged hills, especially in the north of
Britain. It appears in a lot of placenames. According to the Oxford
English Dictionary, it's a loanword from Old Norse, related to the
German word Fels "rock":

1. A hill, mountain. Obs. exc. in proper names of hills in the north-
west of England, as Bowfell, Scawfell, etc.

2. A wild, elevated stretch of waste or pasture land; a moorland
ridge, down. Now chiefly in the north of England and parts of Scotland.

I suspect my associations for this word have been influenced by
Tolkien, who may have revived sense 1, and by reading Old Norse.
According to the OED, English 'moor' originally meant marshland,
related to ON moerr, and mýrr?, both feminine jo-stems, and
English 'mere'; the present meaning of 'moor' in English may have been
influenced by the etymologically unrelated ON mór (gen. mós, pl. móar).

I wonder how well the meanings match of the Icelandic and English
cognates heiðr : heath. Where I live, in the east of England, a heath
is upland, a plateau, not as good farmland as the lowland, and
traditionally used for pasture rather than agriculture, but often
cultivated nowadays thanks to improved technology. In parts of
northern England there are hilly areas called wolds (the Yorkshire
Wolds, the Lincolnshire Wolds). In the south-east, the same word
appears as "The Weald". These are all grassy hills. Likewise "The
Downs" or downland (=upland!). The word is cognate with ON 'völlr',
which I think is a grassy plain, and with German 'Wald' "forest" <
Proto-Germanic *walþuz. To me, moor or moorland suggests a hilly area
a bit more rugged, ranging from grassy areas suitably for grazing to
heather and rocky outcrops and peat bogs. You can see pictures on
Google Images of the North York Moors and the Scottish moors. There
are also famous moors in the southwest of England: Dartmoor, Exmoor.