A job pipeline report can give you an overview of your candidates that move across stage categories in your pipeline.
You can either choose to see your default pipeline, which gives you the bird's eye view of all candidates across all active jobs.
Or you can choose specific jobs - where you will be able to see candidate movements across stage categories for that particular job.
The percentages indicate the movement of the number of candidates from each stage, ie, how many candidates made it across to that stage from the previous one.
This report is based on the categories you select while setting up your pipeline as shown below :
More about categories and why they are important here.