* Homonym: A word that is spelled the same as another word but has a different meaning. The word "their" in the phrase is a homonym of "there."
* Their refers to possession (belonging to them)
* There indicates location.
* Homophone: A word that sounds the same as another word but has a different meaning and spelling. While there aren't any obvious homophones in the exact phrase, you could consider the word "place."
* Place can be a noun, meaning a location.
* Plaice is a type of flatfish (though not as commonly used).
Example of how it could be used:
"They left their word in that place, but I couldn't find it there."