The scale length of an instrument is the distance from the nut to the bridge. Scale lengths vary depending on overall size of the instrument: mandolins have a short scale length around 14″, guitars vary but on average are around 25″, and larger instruments like basses have even longer scale lengths.
Scale length affects a number of things, but most importantly it affects where fret locations will be – in other words, where different notes will be found on the strings. Even if your instrument is not fretted, your scale length affects where you will place the slide to find the key chords. Scale length is even more important if you are fretting your instrument — you have to be certain of your scale length before you start marking and cutting fret slots, or your instrument will never intonate correctly.