It's a simple mistake, but make it on a forum, and the internet will never let you live it down.
Their: possession. "It is their car."
There: location. "The car is over there." (Hint: if you can use "here," chose 'there")
They're: contraction "They're my best friends." Hint: Replace "they're" with "they are" every time. If it still makes sense, you did it right.
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