Edge warrior is a term unique to Zombie Escape and is used to describe a player who stands at the edge of boxes, crates, pipes, walls or ladders etc, usually at important choke points where all players gather. The player gets easily infected by zombies as a result and usually will kill all his teammates afterwards. Edge warriors are a main problem regarding the zombie escape gameplay and are often the reason of many fails, eventually making a map continuously unplayable/unable to be won.
Players are usually unaware of their behaviour as an edge warrior and therefor admin will not ban you for this "behavior". However, if you are intentionally being an edge warrior, you are likely going to be kicked or banned. Doing so is highly disregarded by the community and this will also ruin everyone's gameplay experience. Keep in mind that the biggest majority really play zombie escape for fun and pleasure so you gain no support by doing so.
Warning: Some servers have zero tolerance regarding such behavior and can get you kicked/banned even if you didn't do this intentionally. If you want to be sure to avoid this, read the rules of those possible servers!