PF

Is it b/c the glass will break?

I get what this expression is supposed to mean metaphorically, but it doesn't make sense at all. It's a very poor metaphor IMO.

I think the analogy of the glass house ,refers to the fragility of our egos. By criticising others, and thereby damaging their ego, we leave ourselves open to retaliation, which could possibly leaveus shattered ,like a glass house.

metaphors by definition are not literal so in that way all metaphors don't make since, but their implication does.

it means that you shouldn't be saying things about others when you yourself are not doing so well.

They can throw stones, but they should step outside first.
Frankly, it is a stretch as far as metaphors go. It leaves you wondering if the problem is the retaliation from those that the stones are thrown at, or the problem of throwing stones from within your glass house. I don't get it either, but somehow the message seems to get through.