, anterior is a binary feature
that marks whether a sound is made with the tongue
on or forward of the alveolar ridge
(these are [+anterior]) or behind it ([-anterior], sometimes called posterior).
The term was introduced into phonology by Chomsky and Halle in their The Sound Pattern of English (1968), commonly referred to as SPE. In their original view features were all binary and applied to all sounds, so everything was either [+anterior] or [-anterior]. This overlapped with another feature [coronal], and the labial sounds p b f v m were specified as [+anterior, -coronal].
A reformulation of the theory for greater explanatory power now has features only present when they're needed, and the [anterior] feature is now a subdivision of the [Coronal] place of articulation. Other sounds, such as labials, just don't hold any value for these.
Anterior is still binary. Coronal sounds that are [+anterior] include the dental series (English th) and the alveolar (t d n l s z). The [-anterior] sounds include postalveolars such as English sh ch j, the retroflex sounds of Indian languages, and most varieties of English r.
To distinguish dental from alveolar another binary feature called [distributed] is needed, cutting across the +/- anterior feature.