Simplify the following text for a Grade 13 native English-speaking person.
Original: They are often referred to as defencemen , defensemen , D , D-men or `` blueliners '' ( the latter a reference to the blue line in ice hockey which represents the boundary of the offensive zone ; defencemen generally position themselves along the line to keep the puck in the zone ) .
Simplified: They are often calleddefencemen , defensemen , D , or `` blueliners '' ( a reference to the blue line in ice hockey which is boundary of the offensive zone ; defencemen generally stay along the line to keep the puck in the zone ) .