These one-foot-wide lines travel the width of the ice at 85 ft long. They are 60 ft from the closest goal. The distance between the two blue lines (the neutral zone) is 50 ft and the distance from the blue line to the back of the boards on either side (defending or attacking zone) is 75 ft.
These lines do more than act as a divider on the ice, they are also key in calling offsides penalties. There are two officials called the linesmen that survey the ice and watch for offsides violations. The puck must enter the attacking zone and pass the blue line before the attacking player does, if not the attacking team will be charged an offsides penalty.
In a delayed offsides penalty, the player is in the attacking zone before the puck, but the puck is shot and controlled by the defense. This is still an offsides penalty, but all players can tag up to get the penalty waved off. Tagging up is when all players on the offending team use the blue line to go back into the neutral zone before going back into the attacking zone to avoid the offsides penalty.
Once the penalty is called by the linesman, there is a face-off at the nearest face-off circle. The blue line belongs to whatever zone the puck is in at the time, so when moving into the attacking zone, the puck must completely pass the blue line before a player's skate does.
It can also be known as the point. This is an area where players position themselves so that they're able to either move into their attacking or defensive zone easily without being too far away. Most often defenders will play at the point just inside the attacking zone just in case the forwards need help.