They're should be used when saying they are because the apostrophe is to show the letter is missing for contracting they and are together.
Their should be used when saying a possession.
There is used for referring a place.
2 examples for each homophone
Over there is a concrete wall.
There is a shop around the corner.
They're going to the pools.
They're going not you.
It's their couch.
DON'T TOUCH IT!! It's their chocolate.