Earth's equator is midway between Earth's North and South Poles. If we imagine this line extended out into space, that is the celestial equator.
Just as Earth's latitude measures how far an Earthly location is north or south of the Earth's equator, declination measures where a planet is, north or south of the celestial equator. When a planet crosses the celestial equator, it is at 0° declination.
The Celestial Equator is the baseline measurement of Declination, similar but completely different from the Ecliptic which is the baseline measurement of Celestial Latitude.
Note: Earth's geographic equator and latitude are completely different from Celestial Latitude.