A saddle is like a backbone of a guitar, it is hugely responsible for perfecting the tuning of your instrument. It holds the strings above the bridge and guitar; it helps in muting the vibration of the string, thereby minimizing the buzz on the string. A saddle is a thin strip of bone (ivory or plastic) which is placed on or parallel to the bridge. It is most commonly used in an acoustic guitar.
A saddle affects the core attributes of a guitar; the string length and the intonation. The height of the saddle affects the string height/action; the material of the saddle affects the tone of the instrument. Guitar saddle adjustment is carried out to adjust the height of the string and the intonation.guitar saddle
The first step is to determine the string height of the instrument by measuring the distance between the top of 17th fret and the bottom of the string. If the string action is high, then the bridge saddle needs to be lowered by rotating the Allen key clockwise. And if the string action is lower then the bridge saddle is raised by rotating the Allen key counter clockwise.
The next step is to determine the intonation of the guitar, since each string sits on a saddle, the positioning hugely affects the length of the vibrating string. Play the 9th and 12th fret harmonics and listen carefully; if the tone is sharp then move the saddle backwards or away from the fret board. Whereas if the tone is flat then move the saddle towards the fret board. In doing so, make sure that you make the adjustments in small increments for better results.
A quick tip: saddle screws tend to get loose and fall out, you can try to put a very little amount of white glue on the threads. The glue will not stick to the metal very well, but it will get between the threads of the screws and saddle, henceforth holding the screws intact.