This is the 944th most frequent Icelandic word.
undanfarna
"Undanfarna" means "previous" or "preceding" in English.
Here 'undanfarna' is used as a descriptor to denote the recent days in plural, modifying the noun 'daga'.
Ég hef verið að lesa mikið undanfarna daga.
I have been reading a lot during the past few days.
In this sentence, 'undanfarna' describes the recent months, again in plural form, linked to the noun 'mánuði'.
Húsbyggingin hefur verið í miklum framförum undanfarna mánuði.
The construction of the house has seen much progress over the past months.
'Undanfarna' here is applied to a feminine singular noun 'viku' in accusative case, showing the recent time frame.