On guitar music, what does a number and a horizontal line underneath the notes mean?

Question

In the picture we can see two cases of horizontal lines underneath a group of notes, on the first one there is only one line with a number 1 beside it and on the second one there are two lines, the longer one with a number 1 and the shorter one with a number 3.

So what do these lines mean? What do the numbers beside them mean? And what does their length mean?

Thank you

Answer 1

It means that you should leave your first (and later third) fingers where they are throughout the underlined measures (or as long as the line reaches), because you will need them again on the same frets, and this minimizes unnecessary left hand movement. These numbered lines are recommendations and are purely for didactical purposes.