Their is the possessive form of they. There is an adverb (meaning "at that place") and a pronoun used to start a sentence. They're is a contraction of they are.
Examples:
There are three children in our family. They're all girls. Two of them are sitting over there. Their sister is at home.
Practice:
(a) _____ are geese in the garden.
(b) _____ nibbling the roses.
(c) _____ honking can be heard for miles.
