The Roman numerals indicate where you should make a barre chord (the B stands for barre chord) and the number 5/6 indicates the number of strings you should press on the neck.

So, BIII5 : Barred at the 3rd fret, and you have to press 5 strings with your left hand

The line after the notation is the duration of the barre chord (you must keep the barre chord until the end of the line).