The use of verbs in sentences where the subject is acted upon by the verb instead of doing the action, which is the normal case.
For example:
Active voice: Arthur sought the Holy Grail.
Passive voice: The Holy Grail was sought by Arthur.
The normal giveaway for passive voice is the use of temporal adverbs and clauses such as "was", "has been", "would have been", etc. before the verb.
Passive voice is considered acceptable for research papers, because of its impersonal feel. Excessive use of passive voice in prose, however, is considered bad form, because it can result in confusing sentences and can make prose seem dull and uninvolved.
Use of passive voice is the most common form of grammatical "error" flagged by most modern grammar-checkers.