Originally posted by sherif_shaabanI'm not sure what this means. If you mean the distance between the strings and the frets, that is something called the 'action' of the guitar.
... a friend of mine mentioned something about the distance between the strings and the bar...
James spoke true with his advice. Classic guitars need to have more space between the strings and the frets (higher action) because the nylon strings move more than steel strings when they vibrate. The space beneath an unfretted (open) string should be somewhat greater at the body end of the finger board than at the nut. The space under the thickest string should be a little more than under the thinnest string.
If you hold a string down at the first fret and the fret closest to the bridge, there should be some space between the string and the frets between these two points. This shows how much 'bow' the neck has.
Take note of these dimensions when you are trying out various guitars. This will help you build up some clear ideas of what you prefer and what you wish to avoid.
Happy hunting!