The implications were that it was the dark side. Shortly after this scene, we have Snoke and Kylo Ren conversing about her and noting that the dark side "can get her what she wants", implying it's down that hole.
What she wants is to know who her parents were
Out of universe, the scene is clearly directed at fans, who would recognize the infamous scene on Ahch-to as being where the dark side was. As to why the need for the scene, consider the setup
Luke has confirmed Kylo's version of events. That he, briefly, wanted to murder Ben Solo, and Ben used the opportunity to strike back. Because he didn't tell her the whole truth, Rey rebels against Luke, going to the "dark place" that was drawing her in her earlier vision. So she goes and finds the mirror wall.
It's obvious that she's being tempted by the dark side in some fashion, and it's heavily implied later that she actually got the answer she wanted. Kylo explicitly calls her on it, telling her he knows who her parents were, and that she "knows it's true", before spilling the beans.
It's never spelled out why she didn't turn, or if that event changed her alignment in any way. All we know is that, after meeting with Kylo Ren again, she rejects being a Sith.