A YouTube video I watched recently claimed that summoners might have been considered a bad class in second edition. However, it doesn't seem like that at a glance - there's opportunities for various creatures and beings to stand by your side, such as a plant monster kinda like Blossomon, a stegosaurus, dragons, and so on. Also they come with a nine level magic collection on top of the Eidolon - and the Eidolon can learn spells too with the correct feats. So you could make a druid like character with lots of magic but their main aspect being able to summon a giant sunflower, as a result of acquiring lots of Druid archetype feats for magic casting and feats related to the Eidolon casting magic themselves. Or you could make a stegosaurus man who summons a powerful stegosaurus that knows a lot about nature and fights for them. Sounds like a cool class to play, but I haven't tried the game much to see how any of this would work in gameplay.
Whoops, I think it's actually used to describe Warlocks in 5e, but it's casters that lose their lower level spell slots as they gain higher ones. So far, only Magus and Summoner do this.
Basically it's another term for what is normally called "bounded casting" by people of Pathfinder Second Edition.