Gratuitous display of characters in skimpy clothing, or none at all, under the assumption that it will attract or “reward” viewers. The character is usually female, though fanservice of male characters is far from uncommon.
This is the best kind of fanservice because it’s sexy without being oversexualized. If you look at what’s going on in each of those scenes, there’s nothing overtly sexual happening, either from the character’s or from the viewer’s perspective. The characters aren’t in a sexual situation and the showrunners aren’t portraying it in a sexual way (ie, slow pans along someone’s body, lighting, music, etc). It’s still sexy, because these are some amazingly gorgeous people, but they are, even in those moments, more than sexual objects.
(Of course I also have to give kudos for there being as male fanservice as female fanservice.)