When someone has become good at something, they are proficient. After all those hours playing video games, you must be very proficient at them.
Proficient comes from the Latin for making progress, so if someone is proficient, they have made so much progress that they've become good at something. Use proficient to talk about improvement through practice instead of natural ability; even a person with no inherent skill at language can become proficient in Latin. If you're proficient at tennis, you're good, but it doesn't imply the kind of mastery for which the expert is used.