Dignity
In plain words
Whether a planet is somewhere it finds easy or somewhere it finds hard. A planet at home does what it does well; a planet out of place still does it, but against the grain.
In the tradition's terms
A planet's condition by sign: rulership, exaltation, detriment, fall.
In depth
The essential dignities are older than the horoscope as we draw it. Ptolemy sets out rulership and exaltation; the later tables add term and face, which most modern practice quietly drops.