You are invited to journal because jotting things down in the moment (or soon after), and then looking back at them is proven to be a very helpful way for a lot of people to discern their vocations.
Journaling come in lots of formats - words, images or sounds. Maybe in a notebook, on a mood board (e.g. paper collages or Pinterest), vlogs (on a webpage or on your phone), or other options.
When you register as an Explorer you will be sent a film about journaling.