First of all, I would like to say that most acoustic guitars do not have adjustable saddles, the solid bridge is fixed and therefore, if the intonation is out, you may just have to live with it.
The intonation refers to the guitar being in tune with itself. The frets of the guitar have been placed at a specific place depending on the guitar’s scale. A guitar’s scale length (or any fretted instrument) is the distance between the nut (or the open string) and the bridge’s saddle. In order to accurately determine an instrument’s scale length, measure from the front edge of the nut to the center of the 12th fret and double the result. This measurement not only determines where the frets are placed for proper intonation, but also has a profound effect on string tension and tone. I will give you an example: