it's the equivalent to is/are.
My name is Kevin-Watashi no onamae wa kevin desu 私のお名前はケビンです。
That is a cat. Sore wa neko desu. それは猫です。
not to be mixed up with aru/iru (there is)
he is there right now.
Ima asoko ni kare wa iru. 今あそこに彼はいる
cutting it off, i think, is usually used for casual talk. The casual version of -desu is -da
I hoped this helped in some way
