Kathyan....in simple terms.......this is not a diagnosis, but a description of symptoms that have many causes.
The nerves as they pass out from the neck to the arm have to run the "gauntlet" of a tunnel that is formed from tissue that is connected to the spine and the 1st rib........this tunnel contains the nerves, blood vessels and veins on their way to the arm. Normally there is plenty of room for them all, but in certain conditions, the tunnel gets a bit crowded giving symptoms in the arm........thoracic outlet syndrome symptoms.
The most commonest cause is a thickening of this band, although there are far rarer causes.