* Checked: This is the past participle of the verb "to check." It acts as an adjective in this phrase, describing the state of something (e.g., "the checked bags").
* In: This is a preposition showing location or position.
* Please: This is an adverb modifying the entire phrase, adding a polite request.
Overall: "Checked in please" is a phrase functioning as a polite imperative (command) or request.