Learn Icelandic: Hvar eru vinsælu hlaðvörpin? - Where are the popular podcasts?

The noun it describes, hlaðvörpin (the podcasts), is definite (it has the equivalent of "the" attached to its end). When an adjective comes before a definite noun, it uses the weak declension. In the plural, this weak form ends in -u.

Do I need to know the gender of hlaðvarp to choose the right weak plural ending for vinsælu?

No, and that makes things much easier! In the weak plural declension, adjectives end in -u across all three genders (masculine, feminine, and neuter) in the nominative case. You just need to remember this one ending for plural definite adjectives.

What is the singular form of hlaðvörpin, and why does it have an 'ö' here?

The singular form is hlaðvarp (podcast), which is a neuter noun. When forming the plural, the vowel 'a' changes to 'ö' due to a very common vowel shift rule. Then, the neuter plural definite article -in is added to the end to mean "the podcasts".

Is hlaðvarp a newly invented word?

Yes! Icelandic famously creates native words for new technology rather than borrowing English ones. It combines hlað- (loading) and -varp (casting, just like in sjónvarp for television) to make the word for podcast.