Progress is defined by the amount of activities done that make up the Course or Learning Path you've selected.
For a Course:
A course is made up of multiple lessons and assessments. Each activity has a pre-defined expected amount of time in minutes (e.g. based on the length of a video, or a standard time per question in an assessment). Progress is calculated by the time expected for the number of activities completed, relative to the total time expected for all the activities in the course. So if the course has a total time of 60 minutes, and a learner has progressed through activities equalling 30 minutes, the progress % will be 50%.
For a Learning Path:
If a Learning Path has 5 Courses, and a learner has completed 4 courses, the progress will be 80% (4/5).