| If you mean tempered pitch, that's a big question, involving string physics maths and hours of fun studying to understand it completely. A short answer is that a note is made up not just of the fundamental tone (the pitch of the note) but of a series of overtones above it. You might know these overtones as the harmonics your string can produce, and with practice you can also hear them within an ordinary note, especially when you bow closer to the bridge. A few hundred years ago we evolved a system of music based on the division of the octave into 12 equal parts (these are semitones) so that we could play music in and out of 12 different keys. The notes that this equal division produces are closely related to the natural overtone series but are very slightly different. This is called equal temperament, as opposed to the pure or natural pitch of the overtone series.
If you are playing with a piano or any other fixed pitch instrument you need to know equally tempered pitch. On the other hand ensembles without fixed pitch instruments eg choirs or string ensembles, may have to obey equal temperament if their music is going through different key centres, but also have the freedom to revert to pure intervals for more musical sounds. So you need to be aware of natural pitch as well.
Hope this helps, as a general introduction to the subject,
nick | 
