Nouns and adjectives in Czech change their endings to express different concepts and different relationships between words. These changes form a system of grammatical cases. It can be somewhat compared to how English uses prepositions to achieve the same.
Kráva is the basic, dicitionary form - it's called the nominative case and it is used for the subject of a verb. Krávu is the accusative case which is mainly used for the object of a verb.
It might sound confusing, but don't worry; you will slowly learn more about different cases in different skills in the tree.
You may wish to consult the tips & notes sections of every skill (unfortunately available for web only) or Wikipedia as well. Or look at this article that I have found just for you: http://mluvtecesky.net/en/grammar/cases
Hope it helps and happy learning! :)