A design can be anywhere from impossible to manufacture a single unit to easily manufactured by the millions with very low defects.
Manufacturing feasibility is 'where' along that spectrum a particular design resides. Many things must be taken into consideration to determine manufacturing feasibility.
Are the materials used in the design common/standard?
Are the processes required to form the materials common/standard?
Do the tolerances necessary for the design to work exist within the capabilities of the manufacturing equipment?
Is there proper access for tools/equipment used for assembly?
The list goes on...