From here it says that
Techniques to solve instance segmentation can be roughly grouped into two categories: proposal-based methods and proposal-free methods. In proposal-based methods, a set of object proposals and their classes are first predicted, then foreground-background segmentation in each bounding box is performed. The proposal-free approaches exclude the step of proposal generation.
What is "proposal" in this context? Also, how to "first predict their classes"? There is not much explanation about this topic on the internet and I would appreciate it if someone could explain the differences.