"Too many men on the ice" is a bench minor penalty in ice hockey. Basically, in an NHL game, or games in most hockey leagues, there must not be no more than six players from each team on the ice at any given time (unless in overtime). Usually this is five skaters and one goalie. And usually the five skaters consist of three offensemen and two defensemen. The three offensemen are considered to be on a scoring line - one at left wing, one at center, and one at right wing.
Occasionally - and this is sometimes a coaching gaffe - a player on the bench will jump the gun during a sloppy scoring line change and come out onto the ice before the player he is replacing is behind the bench (which would briefly put seven players on the ice). Too many men on the ice is called when the offending player touches the puck or gets involved in the play somehow. It is usually OK to jump the gun as long as you don't interfere with play.
This penalty is a two minute minor and results in the penalized team being shorthanded for 120 seconds and the other team being on the power play. See those nodes for more information.
One infamous recent example of this penalty affecting the outcome of an important game was Game Six in the 2003 Eastern Conference quarterfinals between the Washington Capitals and Tampa Bay Lightning. Jason Doig came off the bench too early during the third overtime. During the ensuing power play for the Bolts, Martin St. Louis scored, thus ending the season for the Caps and giving Tampa its first-ever playoff series win.