All forces are vectors, which means they have both a strength and a direction. For example, lift is the force created by the wings and its direction is perpendicular to the relative airflow. In straight and level flight, this direction is convenient as it is directly opposite the downwards force of weight. However, as soon as the aircraft is banked the lift vector is tilted and it is no longer opposite in direction to weight.
For convenience, we can break this vector down into two components. One vector that is directly opposite weight and one that is perpendicular. We now have a vertical component of lift and a horizontal component of lift. To keep this aircraft level, the total lift force needs to be increased until the vertical component is again equal to weight.
Any force can be resolved into two components in this manner.
We can also reverse this procedure to combine two seperate forces into one resultant force.