A player "at E-1", or "elimination minus 1" is one vote short of the majority votes needed to eliminate and end the Day, and can be eliminated with a single hammer vote.
"E-1" is the most commonly used variation of the "E-#" notation, and is usually seen in votecounts to warn players of this status. The # may be replaced with any other number to signify that it will take that many more votes to eliminate a player.
It is generally encouraged not to put a player at E-1 unless you are certain that they are scum, and that you have accomplished everything necessary for the Day. Doing so gives scum and impulsive town players a chance to quickhammer, resulting in no chance for a player to claim or not providing the chance to debate a claim, cutting off discussion for the Day, and so on.