When the word outside is used as an adverb it is giving extra information to a phrase. It means that something or someone is situated outdoors or on the exterior of something.
Therefore the phrase to play the game outside means to play a game, such as rugby or tennis, outdoors where you are not sheltered by walls or a roof, for example.
There are many indoor and outdoor games. Indoor games such as bowling or snooker are played inside. They are usually played inside a games hall or a bowling alley for example.
Whereas outdoor sports such as skiing or golf are played outside. Outdoor games can be played on fields, ski slopes, outdoor stadiums, etc.
