Kuroo and Kenma both play together on Nekoma's volleyball team. Kuroo is the captain while Kenma is his setter. They work together well and are quite the pair to beat once put up against them.
They have known each other since childhood. Even though Kuroo is a year older, it does not shift their dynamic in any way. They first met as neighbors and Kuroo taught Kenma how to play volleyball. Kenma is the quiet type but still decided to play with Kuroo. They stuck together ever since going to the same middle school, high school, and joining the same volleyball club.
Though the anime has not caught up yet, in the manga, Hinata Shouyou helps Kenma realize how fun volleyball could be. His whole life Kenma was not extremely addicted to the sport and only played it because it was there. After being shown, he goes to Kuroo and thanks him for getting him into volleyball.
These two are a fairly popular pairing. Their fanon selves do not stray far from the canon universe at all. In canon, Kuroo is somewhat protective over Kenma, but in fanfiction that is doubled. A common storyline fics these two share is when Kuroo leaves for college. Since Kuroo is a year older, he would obviously have to go to college first, leaving Kenma behind at Nekoma. This storyline leads to angst, and if they weren't already in an established relationship, pining. In fanfiction, Kuroo also often nicknames Kenma as 'kitten' because of how much he resembles the animal.
They are seen as a healthy ship by fans but sometimes cause shipping discourse. This ship is part of the only large shipping war in this fandom. People tend to argue over them, TsukkiYama, and Kurotsuki.