Why is polaris a stable sky marker today?
The earth is in constant revolution around the sun and also rotates around its axis. The axis is a line that joins the North and South poles. A star that lies in the same line as the axis would appear to remain stationary while the other stars move around in a circle during the night. Polaris is a star that right now is approximately in the same line as the axis of the earth. As the star is very far away, even while the earth revolves around the sun, the direction in which Polaris lies remains the same in the night sky. Polaris moves around in a very small circle during the night. This makes it a stable night marker, indicating the north direction.
The orientation of the axis of rotation of the Earth is not constant. There is a constant change of about one degree every 72 years. This changes the way stars are seen to be moving in the sky over observations spread across centuries. As a result, Polaris will not remain the stable night marker forever. Its position will be taken by another star in the time to come.
Polaris, the North Star, is a stable navigational marker in the night sky because we always see it in the same position from the northern hemisphere. It's close to the north celestial pole, so it's almost straight overhead when viewed from the North Pole. The rest of the night sky appears to rotate around the North Star due to the earth's rotation. Polaris is one of the brightest stars in the night sky to it's easy to locate.
While Polaris is currently the North Pole star, it hasn't always been. The earth's axis of rotation slowly changes over time. The apparent position of the North Star is slowly moving away from the north celestial pole as the earth's precessing axis sweeps out a circle in the sky, but it will come back around to that position. The period of precession is about 26,000 years so it will take 13,000 years for Polaris to reach its maximum distance from the north celestial pole.