Wikipedia says that a deponent verb is active in meaning, but passive in form. The University of Surrey describes deponency more generally as a mismatch between morphology and morphosyntax. That is, a mismatch between form and function. There is a fairly accessible paper by Matthew Baerman which goes into more details. The active/passive verb contrast…
— Read on rinse.wordpress.com/2010/11/04/v38-minnas/