Vor & Nach are German words meaning the same as English words Before & After. Simply put, they have to do with timing.
Vor and Nach are two of the five fundamental principles that were taught by Master Johannes Liechtenauer and his lineage of masters within the German Tradition of Historical European Fencing.
In a combat situation, these two words describe who has the initiative and who does not. Thus he/she who has Vor is proactive, while he/she who has Nach is reactive - at least for that moment!
The fencer who attacks or offends while the other must counter or defend has Vor and thus forces the other to deal with this onslaught; which limits options to move and act freely - unless the other can gain Vor.
The one who extends an attack by a combination of techniques - e.g. does an Oberhau then either Duplieren or Durchwechseln - tries to keep him/herself in Vor and the other in Nach.
If the other fencer does a correctly timed & executed counter-attack, then he/she starts that action in Nach but gains Vor and thereby puts the one who attacked in the first place into Nach!