The quantity on the item ledger entries is a value that is owned by the entries or transactions that occur such as purchase or sale
while the remaining quantity is the residual value remaining on the line entries that can be used. (inventory)
for example, you buy a pencil with a quantity 10 pcs. then at the beginning you have 10 pcs quantity remaining on the field. then you sell it and take a pencil from that you have purchased as many as 3 pcs. so now you only have a remaining quantity of 7 pcs
I am also a beginner in NAV, sorry if my explanation is less