Pianos have thousands of moving parts that translate the key movement to hammer movement.   Many of these parts are subject to wear and/or breakage.    Many are subject to humidity.   Many common repairs can be done right in the home, including some or all of the following:

• Filing hammers to restore strike points.  Deep grooves or flat strike surfaces decrease the quality of a pianos sound.
• Tight action centers.   These are the flanges on which things move in the piano action.   Most parts of the action operate on pinned flanges.   These action centers can become tight and need to be freed up for optimal action function.  Loose action centers can result in wobbly travel of the parts.
• Tight keys.   The felt bushings on the underside of keys can become tight.   Easing keys is a relatively simple procedure, but careful analysis is needed to be certain that is actually where the problem is.
• Broken hammer shanks.   The best repair is replacement of the shank.
• Broken strings.   Strings can either be completely replaced or repaired using a splice.   The latter is often preferable since new strings require frequent retuning.   Broken copper wound strings are often best repaired by splicing.  It is not a good idea to just do without a broken string.  This can cause hammer and flange damage.
• Squeaks and clicks.   These often require trial and error and a good investigation into when the sound occurs in the keystroke or what induces the sound.   I remember clearly the grand piano with a buzz on one A# and no other key.   The solution was tightening all of the rim bolt screws.voila.the buzz went away.