Learn German: Spanien ist ein beliebtes Reiseziel. - Spain is a popular travel destination.

Reiseziel is a neuter noun (das Reiseziel). Because it comes after the indefinite article ein in the nominative (subject) case, the adjective beliebt (popular) needs the ending -es to show that the noun is neuter.

How do I know the gender of a compound noun like Reiseziel?

In German compound nouns, the very last word always determines the gender. Reiseziel is made of die Reise (the trip/travel) and das Ziel (the destination/goal). Since Ziel is neuter, the whole word becomes neuter: das Reiseziel.

Since this lesson is about irregular superlatives like "am liebsten", how does "beliebt" relate to it?

They look similar because both come from the root for "love" or "like" (lieben), but they are used differently. Beliebt is an adjective meaning "popular" or "well-liked", and it forms its comparative and superlative regularly (beliebter, am beliebtesten). Am liebsten, on the other hand, is the irregular superlative of the adverb gern and is used to describe an action you like doing most.

Why doesn't Spanien have an article like "das Spanien"?