Learn Finnish: He puhuivat tarkasta kuvauksesta. - They talked about the exact description.

It comes from the verb kuvata (to describe / to picture). By taking the verb's stem and adding the suffix -us, it creates a noun that represents the action or result of the verb: a description.

What happens to the word kuvaus when we add the case ending?

Finnish words ending in -us or -ys usually change their ending to -ukse- or -ykse- before you add a case suffix. That is why kuvaus changes to the stem kuvaukse-, which gives us kuvauksesta.

Why do kuvaus and tarkka both have the -sta ending here?

The verb puhua (to talk/speak) requires the elative case (-sta/-stä) when you are talking about something. Because the noun "description" is in the elative (kuvauksesta), the adjective describing it must also match that case (tarkasta).

Why does the adjective tarkka (exact) lose a 'k' in tarkasta?

This is consonant gradation. Adding the -sta ending closes the final syllable, which forces the strong, double kk in the basic form tarkka to weaken to a single k.