A studio is the animation production company responsible for making an anime: organizing staff, animators, directors, and subcontractors to deliver episodes or films. Major series are often co-produced and heavily subcontracted, so a studio credit names the lead production house.
A studio is not a single team. Large studios run several production lines at once, small studios specialize, and nearly every television production draws on a web of subcontractors for in-between animation, backgrounds, photography, and finishing. The studio name on a series tells you who organized the work, and the episode credits tell you who actually did each part.
Studio identity still means something real to fans. Houses develop house strengths, in action choreography, in background art, in adaptation faithfulness, and a studio change between seasons is news precisely because viewers can often see the difference. Tracking which studio made what, and when, is legitimate production history.
What is not content here is the labour story behind the work. Crunch, overwork, and working conditions are serious matters and they are not quiz material on this site; a troubled production's output may be studied, its suffering may not be quizzed. The works are judged as works, which is the same rule this vertical applies to every studio and every creator. Anniversary histories and studio profile books are a small publishing genre of their own in Japan, and they are the sanctioned record from which studio filmographies are drawn.
In use"When a series changes lead studio between seasons, the new studio's name leads the credits and the coverage."