blimp:
Never can get my their's right. Or is it there?
Sorry Blimp! English teacher can't resist!
Their = belonging to them (possessive adjective like my, your, his, her, our), hence also 'it's theirs' (like 'mine')
There = 1. opposite of here 2. indefinable, as in There is, there were, there would be...
and then of course there's 'they're' ...