This is the 983rd most frequent Icelandic word.
erlendum
"Foreign" or "external."
Here, 'erlendum' is in the dative plural form, describing 'ferðamönnum' (tourists), thereby providing their origin/characteristics as foreign.
Erlendum ferðamönnum fannst Ísland heillandi.
Foreign tourists found Iceland enchanting.
In this sentence, 'erlendum' modifies 'stofnunum' (institutions), indicating that these institutions are from abroad or foreign.
Við eigum að nálgast upplýsingar frá erlendum stofnunum.
We should obtain information from foreign institutions.
'Erlendum' here describes 'vefsíðum' (websites), specifying that these websites are foreign in origin or scope.