Let me introduce the erector spinae muscle group.  This group of muscles runs either side of the spine and is responsible for keeping the spine upright and assists in good posture.  It's main job is to extend the spinal column.  To give you an idea of what extension means - if  exaggerated you would be in a back bend.  

Think about your posture at a desk or computer - invariably it's in a forward hunched or flexed position.  In effect the erector spinae muscle group is being asked to spend hours in a position which it wasn't designed for .  Over time these muscles become used to being in "unnatural" position and become tight in a lengthened state.

One way to stretch this muscle group is shown above.  It's quick and easy to do and can be done at your desk.

One step further:  stretching a muscle will provide temporary relief, however to restore correct muscle length and function, strengthening the muscle is vital.  This can be done by doing deadlift strengthening exercises using a barbell and light weights.