The most important aspect is peer review. At least in physics, journals assign your paper to an Editor (a scientist), that may reject it directly if it is not scientific. If it is, they will send it to another scientist to read the work and (a) suggest rejection, (b) suggest accepting the work directly or (c) in the most common scenario accept the paper for publication after some revisions. The editor reads the review and the informs the author of the paper accordingly, and the story iterates until the work is fine for the reviewer. There can be more than one reviewer (a.k.a. referee). The editor is what the journal offers, together with some spell checking service before publication. Editors are payed, and referees only sometimes.

There are notable, noble exceptions known as diamond open access journals, like my favourite: the Open Journal of Astrophysics