what is the difference progressive profiling and pre-fill?

Progressive Profiling is the form level feature in marketo through which you can collect new/additional information from the user every time he fills the form. There will be few standard fields like name and email address that will be displayed always and there will be progressive profiling fields, you set the number of fields visible each time for the fields in the progressive profiling section. So if you have 5 fields in the progressive profiling section and you have set 2 fields to display at a time then everytime marketo will pick the unknown 2 fields from the 5 fields in progressive profiling section and display it on the form with other mandatory fields on the form. So tl;dr progressive profiling will allow you to gather more data from the known user each time he fills the form again.

Form prefills is the way through which fields like email/city etc get filled when the user revisits the lp having the form. The fields get pre-filled using the cookies stored in the browser when the form was submitted for the first time.

For the prefills this link might be useful.

Please post questions in the appropriate space, "Products," not "Beta": https://nation.marketo.com/t5/Products/ct-p/products

I'm flagging your post so it gets moved.