Woodwind instruments play different notes by closing and opening holes to change the length of the vibrating air column inside the instrument. Most holes in the instrument are closed by a pad glued to a key with the exception of some holes that are covered by fingers on certain instruments like clarinets and oboes. When a hole is not completely sealed off from the outside air it is called a leak. An instrument can play with minor leaks and the better the player is the more they can force a leaking instrument to play properly. At some point the instrument takes to much effort to play correctly or cannot play certain notes.
Pads are typically made out of felt covered by animal skin with a cardboard backing. These materials absorb moisture causing them to shrink and swell creating leaks between the pads and tone holes. The skin of the pad overtime will harden and make it more difficult to seal the tone hole with the pad. Pads sometimes will tear creating a leak and sometimes even bugs will eat small holes in the skin and felt of pads rendering the pad useless. Depending on the condition of the pad it can either be shifted around to seal properly or changed if needed.
Corks are used for many things on woodwind instruments and overtime will need to be replaced. Cork compresses and a cork used for key adjustment, neck cork, and tenon joints will overtime loosen or get thinner and need to be replaced. The glue that holds the cork will also fail in some cases and the cork will fall off and need to be replaced.
Typically woodwinds need an adjustment once or twice a year to maintain a reasonable amount of sealing and functionality of the keys. If certain notes are getting hard to play, keys are making loud clanking noises, pads are falling out, or keys are sticking then it is time to get an instrument adjusted.