These are towns and castles which you can take over control of. This Mount And Blade 2 Bannerlord settlement guide will teach you the basics of how to take over and manage your own town, as well as how to defend it against invaders.
Taking over the settlements is rather difficult, but keeping control and developing them further is also a big challenge. This means managing the many settlements that you'll be conquering along the way. You're also be able to take on the role of ruler in your own kingdom. It's not all just battles in Mount And Blade 2 Bannerlord.