Hybrid publishers are a relatively new concept, and there are a few different ways that they operate. In general, hybrid publishers are a mix between traditional publishers and self-publishers, offering authors a greater degree of control over their work while still providing some of the advantages of working with a publishing house.
One of the most common models for hybrid publishing is for the author to pay a fee to the publisher in order to cover the costs of publication, such as editing, design, and marketing. The publisher then takes a smaller portion of the royalties than they would with a traditional book deal. This can be a great option for authors who are looking for a high degree of control over their work, but who still want the benefit of having a publisher handle some of the more technical aspects of getting their book into the world.
Another common model is for the publisher to provide some or all of the upfront costs of publication, and then take a higher percentage of the royalties. This can be a good option for authors who may not have the funds to cover the costs of publication themselves but who are still interested in having a say in how their book is produced.
There are a number of different hybrid publishers out there, and it's worth doing your research to find one that's a good fit for you and your project.