A lot of native-speaking and ESL English students have trouble using there, their and they’re. These three words are homophones - words with the same sound, but different meanings. (Other examples of homophones are to/too/two and ate/eight.)
There has a couple of meanings. It can mean a specific place, “Don’t step there by the edge of the cliff!” And it can also be said with a “to be” verb (am, is, are, was, were), “There are so many beautiful countries in the world!”
Their is a possessive pronoun. A pronoun takes the place of a subject. For instance, “Mary and Barry run” can also be said “They run.” To possess is to own, so a possessive pronoun is a pronoun that shows ownership. “Jeff and Jeesun are careless. Their tent is on fire!” Their takes the place of Jeff and Jeesun and shows that Jeff and Jeesun own the tent.
They’re is a contraction for the two words ‘they’ and ‘are.’ A contraction brings two words together to make one word that is easier to say and faster to write. So, “they are walking home” means the same thing as “they’re walking home.”