Refers to the instrument being in tune along the fret board. An easy way to check the basic intonation of a guitar is to hit the 12 fret harmonic and compare the pitch with a note fretted at the same fret. If the fretted note is sharp that means the string needs to be lengthened. If flat, it needs to be shortened. Over time the correct intonation of an instrument shifts and would then be in need of resetting. If not reset, your instrument will not play in tune.
High String Action
A good instrument can be hard to play when the strings are too high off the fret board and if too low will cause the strings to buzz. To correct this requires a simple adjustment and is well worth doing. It is part and parcel of a good set up and makes playing the instrument much more enjoyable.
No matter what type of stringed instrument you play the nut is a very important part of the overall tone and playability of the instrument. There are many different types of nuts one can use, I myself prefer to use a bone nut as I feel this transfers both sound and tone very well.
String buzz is a common problem on all instruments. There are any number of reasons for string buzz. Fixing the buzz is a process of elimination.
Certain parts of any instrument are more prone to wear and tear than others, where the finish on the instrument begins to wear off in places. This can be resprayed and blended in or re-oiled. This is important as it stops moisture getting into the wood creating a greater problem.