I think it’s being and feeling safe with the other person to be able to really be yourself around them and them the same around you too.

Romantic or platonic If it's Romantic, then when you can truly know the other person threw-and-threw. Like knowing their favorite color or season, and knowing why they like that color or season. For platonic, it's knowing that they'll always be there for you, and you for them. Like if one of you is going through a bad time, then the other should feel like they're actually there.