It's one of the two coordinates used in the equatorial coordinate system, which is similar to latitude and longitude on Earth but applied to the celestial sphere.

Here's a breakdown:

* Right Ascension (RA): Similar to longitude, it measures the angular distance of a celestial object eastward along the celestial equator from the vernal equinox (a specific point in the sky where the Sun crosses the celestial equator from south to north). It is measured in hours (h), minutes (m), and seconds (s).

* Declination (Dec): Similar to latitude, it measures the angular distance of a celestial object north or south of the celestial equator. It is measured in degrees (°), minutes ('), and seconds (").

Therefore, you can think of RA and Dec as the celestial "address" of a star or other object in the sky.