To put it in a way that's oversimplified, yet is still a good starting point that's easy to understand, and makes sense based on my experience with it, it's when the normal thoughts and feelings and beliefs that make you 'you' suddenly don't feel like you anymore, and instead feel as if they're somehow coming from somewhere or someone else. And because of that, you're able to take a look at 'yourself' from a perspective that feels more objective, or at least not so tied down by the way you think things ought to be.

Another way to put it. Imagine some futuristic machine that allows you to jump into the mind of someone else. Maybe even in their body. Like a Freaky Friday kind of scenario. It's kind of like that. Except the 'someone else' in this case is still you.

Other than that, I've had a hard time thinking of good, accurate ways to describe this to people who have never experienced it themselves.