A rete is also a part of an astrolabe. It is a metal ring, with a smaller ring inside it representing the ecliptic and several points reperesenting stars. There are branches of metal to which the star-points are connected, so it does look a little like a net.
Dates are marked on the ecliptic, representing the path the sun takes in a year. You can see through the rete to the plate, which shows almucanters (lines of equal altitude). To position the rete correctly, measure the altitude of the sun and put the point on the ecliptic disc corresponding to the date over the almucanter for the same altitude, or measure the altitude of a star and put the corresponding point on the rete over the correct almucanter. By projecting a line from the centre of the astrolabe, through the date on the ecliptic, to the circumference, the time of day can be determined. (There is an arm on the astrolabe for doing precisely this.) Thus, the time of day can be determined whether the sun is up or down, provided you know the approximate date and there aren't too many clouds in the sky.