The Playerrank is a combined score of all job levels a player currently holds. Playerrank can be increased through leveling jobs. With all 7 Jobs leveled to a maximum of level 60, the highest possible playerrank becomes 420. For unlocking certain blueprints, as top tier weapons for each job for example, a sufficiently high enough playerrank has to be met in order to be qualified for obtaining the blueprint.
Playerrank also determines whether a lootcrate with a certain level can be opened or not. Keep in mind that your current job level is the determining factor for how much damage is dealt, respectively taken, not the playerrank itself. Experienced players tend to have a higher player rank than beginners who just started to play. In PvP the playerrank is a good indicator how dangerous an opposing player is, as certain job levels unlock passive status boost that can be leveled with element x.