What are they?

Astrolabes are physical, mathematical, interactive models of the stars (including the sun) as seen from earth. Many of them are also art objects bearing expressive decoration.
Astrolabe made by Muhammad Khalil ibn Hasan Ali; Iran, 1100 AH (1688-1689 AD)

What are they used for?

The main function of astrolabes was timekeeping. They could be used for a wide variety of other other position-related calculations, but the major use was determining the time of day.

How do they work?

The basic principle is that the solid parts of an astrolabe contain graphs, and the moving parts allow the user to enter data into those graphs and read the corresponding output. Most commonly the input would be the current position of the sun during the day or a star at night, measured by using a whirling ruler (alidade) and the protactor-like markings at the back of the astrolabe. For some calculations, this measurement might need to be entered into another graph on the front of the astrolabe. The most common output sought would be the time of day – more specifically, how much time had passed since (or was left until) sunrise/sunset.

It is worth remembering that the way we track time is ultimately derived from the position of the earth relative to the position of the sun – both over the course of the day and over the course of a year. Additionally, in parts of the world that did not have much seasonal variation (such as the deserts of the Arabian Peninsula), the positions of particular constellations and other asterisms were crucial for tracking time over the course of the year.

How old are they?

While presumably invented in Greece around 150 BCE, the oldest surviving astrolabes known to academia are associated with 9th century CE Iraq/Persia. But astrolabes remained in widespread use until the end of 17th century in Europe and until the 19th century in the Islamic world. A few modern folks make them, too.

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