What does 100% Pensum mean?

I am very interested in one job post, but not quite sure what "100% Pensum" means. It is a practikumsstelle (80%) at a non-profit organizazion. Can anyone explain, please? Thanks in advance!

Lohn: CHF 2‘730.- brutto bei 100% Pensum

CHF 2'730 is the full-time wage.

If you work 80% then you will gross CHF 2'184