What is 'in progress' or 'I know'?

For each concept, you can set if it is 'in progress' (you are studying it) or if you know it by clicking respectively, 'in progress' or 'I know'. The buttons are just under the name of the concept.

The list of all concepts that you set as 'I know' can be found "Top right picture profile" > "Knowledge"

The list of all concepts that you set as 'in progress' can be found "Top right picture profile" > "In progress"