Construct 2 offline

Are Construct 2 games only restricted to being played online or they can be played offline?

Depending on platform you can export them to an offline-capable state. Just check the Export screen to see what's available for what platform.

If you export using Node-Webkit (now called NW.js) the game can be played on Windows, Linux and OSX offline

also, web games done with C2 are stored in the cache of the browser and effectively work offline when they have been played at least once.

All Construct 2 games work offline. Even when publishing to the web, it uses HTML5 AppCache to store the entire game, so it can still be browsed to and played without an internet connection. This is especially useful on mobile if the user does 'add to home screen' with the game from the browser.