I think it's just being there for someone to turn to if they need somebody to make them feel heard and safe. There are further steps someone can take that would also count as support, but yeah, i think just being there and caring enough to earnestly listen is the foundational definition in my book.
There's no ulterior message here, just a general statement close to my heart, but one thing i think is very definitely not part of support is mind-reading. Some people seem to think if you care, you should be able to intuitively sense whenever someone needs anything, and go all the way to provide that without being asked. Nope, sorry, that's just not realistic. They can pick up a vibe maybe, but they can't really know unless you say something. If you truly want someone to support you, you need to demonstrate the trust to tell them that you need that support.