A "push" would be something that may prevent you from saying yes to something or prevent you from doing that option, it pushes you towards the other choice. A "pull" makes it easier to make that choice, it pulls you in that direction (metaphorically). Pull factors affect those who migrate because these pull factors may be better jobs and better homes for their family which would pull them towards migrating. Something that may push them would be the risks of getting caught and arrested or even worse.
Push factors “push” people away from their home and include things like war. Pull factors “pull” people to a new home and include things like better opportunities. The reasons people migrate are usually economic, political, cultural, or environmental.
