The superlative is how German says "the best", "the biggest", "the most beautiful" — and it follows a tidy pattern: am -sten when used predicatively (am schönsten) or der/die/das -ste + adjective endings when it sits in front of a noun (der schönste Tag).
Most adjectives take an umlaut on the way to the superlative (alt → ältesten, groß → größten), and a small handful are irregular — gut → besten, viel → meisten, gern → liebsten. Once you've worked through these, the comparative exercises make a good companion drill.
German Superlative Exercise
The hint shows the bare adjective — type the superlative form with the right adjective ending. Most of these sit in front of a noun, so they take der/die/das -ste + an ending that agrees with the noun.
Irregular German Superlative Exercises
A second round, this time with adjectives that change their stem. Gut → besten, viel → meisten, plus a few that take an umlaut on the way to the superlative (alt → ältesten, groß → größten, hoch → höchsten).