Have you ever smelled something that evoked a really vivid memory? Where every detail of a past experience was suddenly brought to the foreground in your memory? Maybe a certain kind of perfume reminds you of a past girlfriend or something like that.

In my experience, that's what people are talking about when they talk about "acid flashbacks". It's just that you tend to form very strong memories while tripping. It's not dangerous and it's not something unique to psychedelics.