What the Qibla is
Allah commanded the Prophet ﷺ and the believers to turn their faces toward the Sacred Mosque in prayer, wherever they are (Quran 2:144). The Prophet ﷺ first prayed toward Jerusalem (Sahih al-Bukhari 4486), and when the direction was changed to the Kaaba, the believers at Quba turned around in the middle of the Fajr prayer as soon as the news reached them (Sahih al-Bukhari 403). The Kaaba is the focus of this direction. Muslims all over the world form circles around it.
Facing the Qibla is a condition of the prayer for a person who is able to do so. In fear, Allah allows prayer on foot or riding, in any direction (Quran 2:239). The direction is also used for other acts, such as the burial of the dead.
For a person who can see the Kaaba, the duty is to face the Kaaba itself. For a person far from Makkah, most scholars say that facing its direction is enough, while the Shafi'i school stresses aiming at the Kaaba itself as well as one can. It is good to learn the Qibla of your home, your workplace and the places where you often pray.