Kas, as taught in one earlier lesson, means who or what:

Kas jis yra? Who is he?
Kas tu esi? Who are you?
Kas skambina? Who is calling?

Kas has cases too. They look exactly like tas (that):

Do you see that kas doesn't have feminine (ta) and plural (tie, tos) forms? This means that the same forms are used for feminine and plural words:

Kas eina? Ta mergaitė eina. Who goes? That girl goes.
Ko reikia? Reikia kreidos. What is needed? Chalk is needed.
Kam patinka šitas autobusas? To whom appeals this bus?
Ką perki? Perku traukinį. What are you buying? I am buying a train.
Kuo važiuoji į Lietuvą? Į Lietuvą važiuoju traukiniu. By what are you going to Lithuania? To Lithuania I am going by train.
Kame yra mano knyga? In what (where) is my book?

Kame is usually changed by kur (meaning where):

Kur yra jos knyga? Where is her book?
Kur eini? Where are you going?