It's a bit of both, but it's mainly about experience rather than knowledge. A senior is expected to be able to handle anything that comes up during the course

It's a bit of both, but it's mainly about experience rather than knowledge. A senior is expected to be able to handle anything that comes up during the course of a project (including when things are on fire), to be able to delegate, to be able to mentor juniors and quality-control their work. Juniors are expected to be learning as they go (to a greater extent) and likely to make mistakes or need help now and then. In particular, a senior will know when they have something wrong or it's not good enough. A junior is reliant on others to tell them what's expected in a given situation. If you are a recent graduate you are, by definition, a junior. After a few years you might have the experience necessary to become a senior, if you have earned the trust of your peers, especially those in charge of that decision.