Circular motion

Constant altitude - is to keep the problem simple. If say it lost altitude -it would gain speed. So the problem has been described as a plane turning at constant speed.

The banking - means the plane tilts. To corner, the plane needs to fly along a curved path (part of a circle). To make the plane follow a curved path (rather than straight - Newtons 1st law) there must be a sideways force pushing the plane so it doesnt go straight. The way the plane gets a sideways force is to tilt or bank. Untilted, the lift from the wings is vertically upwards. When tilted the lift is perpendicular to the wings. This means it is mostly upwards but with a sideways component - this sideways element is what causes the plane to turn.
Pete

thanks pete but i don't get how cars can just turn but planes can't

Not sure, but I'm guessing it's got something to do with Friction? The friction provides the centripetal force for cars when turning

When a car turns it's the friction between the road and the tyres that acts as the centripetal force. When an aeroplane turns, it's a component of the lift that acts as the centripetal force. In unbanked flight lift acts straight upwards. When the aeroplane banks, the direction of the lift banks with it, hence creating the centripetal force. Just yawing the aeroplane (turning the nose to the left or right, like turning the front of a car left or right during a turn) doesn't cause the aeroplane to change direction, it continues straight but with the nose pointing in a different direction to the direction of motion. I hope that helps