It's more positive confirmation than simply meaning "it is"
Such as in "daijobou desu ka", desu there means "i am".
In the same way that "no" gives possession of an adjective to a noun in the same way as our "of", and "nai" turns a statement negative such as in "hanase" (let go) to "hanasenaide" (don't let go).

It's a way of answering a phone in Japanese, desu is something you end a sentence with in Japanese