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
