Russian has six grammatical cases. Each case changes the endings of nouns, adjectives, pronouns, and numerals to show their role in the sentence.
Knowing the rules is not the same as using them. In real conversation there is no time to scan a declension table — the correct ending has to come out automatically. Ruscases trains exactly that, with timed, sentence-based multiple-choice quizzes aligned with the TORFL (ТРКИ) levels.