New door binding on frame
Very often the change of seasons and corresponding temperature and humidity changes result in the house and woodworking moving around. 

The usual fix is to inspect the door frame and door hinges to see of they are secure and installed correctly. If all appears to be in order then the next step is to study the door itself and see just where the binding is and how much. Mark the binding locations with some of the colorful type of painters tape and then demount the door via the hinge pin removal. 

The binding areas are normally handled by taking material off the door via sanding, planing or even trimming with a saw. (Note that this applies for wood material type doors - not steel jacketed doors).

Next is to re-hang the door in the frame and test for fit. Sometimes it will be necessary to make several iterations of trimming the door to get the correct fit. The final fitting should be followed up with a fine sanding to get a finish surface. 

The final step is to re-finish the parts of the door where the trimming occurred. This may be a primer/paint job or possibly a stain/finish job depending on how the original finish was applied.