Hi Gad,
For want of further context (I'm assuming it's the same translation as the payslip one you've asked for help for too), I would guess that 'plafond période' would be the period over which the threshold/upper limit is calculated.
A 'plafond' in the sense of employment and payments generally refers to the upper limit placed on salary deductions for social security contributions, but with no further context, it's impossible to tell whether I'm heading in the right direction with this assumption or not.


Refs:

Le Grand Robert:
plafond de la Sécurité sociale : upper limit on salary deductions for social security contributions