Being extremely selective of who they let in..
ensuring their boundaries

To list a couple of things

It seems to me it's another one of those over used social media catch phrases. It's written so often it literally makes me feel nauseous now. And the same thing with boundaries.

I guess it's sort of a barrier around one's self to not be too influenced by outside things that cause anxiety, etc.. Actually I think it's social media that is causing our anxiety and making us less able to handle as much. Sometimes it may be a way to avoid hard conversations or ignore.