their's是表示“他们的”比如：Their's apples are on the table.他们的苹果在桌子上.
theirs也表示“他们的”只是两者用法不同 比如：Are those apples ours or theirs?这些苹果是我们的还是他们的?
Are those apples ours or theirs?=Are those apples our’s apples or their’s apples?这些苹果是我们的还是他们的?看出来了吧,theirs是用来避免重复!在这个例子中theirs
=their’s apples
说得专业一点,their's相当于形容词性物主代词,后面要加名词,theirs相当于名词性物主代词,所以后面不需要再加名词!
