Explore other star systems. Earn money by trading, carrying passengers, or completing missions. Use your earnings to buy a better ship or to upgrade the weapons and engines on your current one. Blow up pirates. Take sides in a civil war. Or leave human space behind and hope to find some friendly aliens whose culture is more civilized than your own...
Endless Sky is a sandbox-style space exploration game similar to Elite, Escape Velocity, or Star Control. You start out as the captain of a tiny spaceship and can choose what to do from there. The game includes a major plot line and many minor missions, but you can choose whether you want to play through the plot or strike out on your own as a merchant or bounty hunter or explorer.
Official releases of Endless Sky are available as direct downloads from GitHub, on Steam, on GOG, and on Flathub. Other package managers may also include the game, though the specific version provided may not be up-to-date.
Endless Sky has very minimal system requirements, meaning most systems should be able to run the game. The most restrictive requirement is likely that your device must support at least OpenGL 3.