Take a look at the first of Tarrega's studies, which I'm trying to learn.
In this study measures 6 and 13 are marked with a roman numeral (V and I respectively), and a dashed line above a measure. What does it mean?
The "V" means to move your left hand hand up the neck to where your first finger is positioned at the fifth fret. The dashed line means to keep your hand in this position for the number of measures indicated by the length of the dash.
The Roman numerals are to guide you to where on the fretboard your fingers are best placed. Often a barre will help, or even a part-barre. It's not necessarily so that all of the notes will be placed exactly on that number fret, but it gives good guidance.The dotty bit shows how long the tune hovers around that fret, before being replaced with another Roman numeral.