This part of an astrolabe is called the alidade. It is used to take sightings and measure the angle on the mater's scale.
The mater holds the other astrolabe parts and has a degree scale for measuring angles. The latitude plate is chosen for the latitude at which the astrolabe is being used. The rete contains a calender and the stars for the hemisphere of interest. The alidade is used to take sightings of stars and is lined up on the calendar to make calculations. To be a functional astrolabe this horse needs to be combined with the additional parts described above.
There are good sources online detailing the use of an astrolabe so I will not go into that here.