Learn Finnish: Tämä on maailman korkein huippu. - This is the highest peak in the world.

The base adjective is korkea (high). To make it superlative (highest), we add the suffix -in. When adding this suffix, the final vowel -a drops out. That is why korkea becomes korkein.

Why does the sentence use maailman (the world's) instead of maailmassa (in the world)?

While English says 'the highest peak in the world,' Finnish usually expresses this relationship using the genitive case (the -n ending). You can think of it literally as 'the world's highest peak.'

If korkein means 'highest', how would I say 'higher'?

To say 'higher', you use the comparative suffix -mpi, which makes the word korkeampi. Remember: -mpi is for comparing two things (higher), and -in is for the top of the group (highest).

I've heard the word huippu used to describe things other than mountains. What does it mean?

You have a good ear! While it literally means 'peak' or 'summit' in this sentence, Finns very often use huippu in everyday speech as an adjective to mean 'awesome,' 'great,' or 'top-tier.'