이다 is the copula, a verb used to equate two things. The copula is also known as the verb "to be". 이다 is unique among all other Korean verbs because it attaches directly onto the end of nouns.

The opposite of 이다 is 아니다 meaning "to not be". 아니다 does not attach to the end of nouns like 이다 does. Instead it should have a space.