In a game of chance, the outcome depends mostly on a random process such as a spinning wheel, a shuffled deck or a random number generator. Player decisions have little or no effect on the long run result. This is why the house edge in these games is set by the rules and cannot be beaten by skill. Laws in many places treat games of chance differently from games of skill, so the label can matter for licensing and for what is legal.
Example
On a slot machine, the player cannot influence the result after pressing spin. The return to player is set by the machine's design.