At the eleventh hour means at the last minute or at the last opportunity. A less common version is in the eleventh hour. Sometimes it is used as an adjective to describe something as last minute. In this usage it is usually hyphenated like most compound adjectives.
The wording for idiom dates from the Bible. In a parable, Jesus talks of workmen hired to tend a vineyard. The men hired at the eleventh hour worked only for one hour, but were paid as much as those working the entire day.
It should be noted that the phrasing from the Bible is not the idiom itself. The Jewish culture counted the days in 12 hour increments, from sunup to sundown, and the eleventh hour was simply the last hour of the workday.