In a game of skill, better decisions lead to better results over time, even though luck still plays a part in each hand. Poker is the usual example, because players compete against each other rather than the house. Blackjack has a skill element too, since correct play lowers the house edge, but it does not remove it. Whether a game counts as chance or skill in law differs by place, and the answer affects whether it needs a license.
Example
Two poker players of different ability may both win or lose on a single night. Over thousands of hands, the stronger player is more likely to come out ahead.