With an in flight aircraft the direction and altitude of flight is determined by the three rotational axis operating from a center point within the aircraft. All three axis work together to determine the attitude of the aircraft. The three separate axis of movement are pitch which refers to a nose up or down situation, roll indicating one wing up and the other wing down, and yaw which is the nose of the plane going to one side while the tail is going to the other.
Do not confuse attitude, the position of the aircraft with altitude, the height off the ground the plane is flying. Attitude can be described as going along in straight and level flight, or it can be flying through the air in a less streamlined situation such a side slipping while turning. You can be at the same "altitude" but you may be adjusting your "attitude" to achieve maximum air speed, or fuel efficiency, or even passenger comfort.
The tree axis are commonly labeled the X axis which is a horizontal line going the length of the plane from nose to tail, the Y axis, which is also a horizontal line, goes through the aircraft from wing tip to wing tip, and the Z axis which goes vertically through the center of the aircraft from top to bottom.
The X axis, which affects the pitch, is controlled by the elevators located on the tails horizontal flight surfaces.
The Y axis, which affects the yaw is controlled by the rudder on the tails vertical flight surface.
Lastly the Z axis affects the roll, and is controlled by the ailerons located on the trailing edge of the wings.
There are also smaller control surfaces known as Trim Tabs that allow small adjustments to the attitude to correct for wind conditions or flight characteristics of the airplane.
Flight controls for the aircraft allow coordination between the various control surfaces to allow for making turns, and changing altitude in an efficient smooth manner.
When you turn the aircraft there is more involved then just turning the steering wheel one way or the other. It takes a combination of input to all three axis to bank a smooth corner so that altitude "height" is maintained.
By making the effort to place the aircraft in a less then streamlined attitude while performing a turning maneuver the aircraft may more rapidly loose altitude. This is called side slipping and in some instances can be an intentional move to accommodate conditions such as making an approach to an aircraft carrier where there is little or no room for error.
Slipping can lower your speed as well as slide your position back to correct glide scope if your approach is just a little bit off.