Meaning: This sentence describes a group of people engaging in an enjoyable activity in a public area with trees, grass, and other recreational facilities.
Choice of Words: The word "They" is a pronoun that refers to a group of people. "Play" is a verb that describes an activity done for enjoyment. "Outside" is an adverb that indicates a location not inside a building. "In" is a preposition that indicates a place or location. "Park" is a noun that refers to a public area with trees, grass, and other recreational facilities.