ELI5: What even is the electrical grid?

Every time these kinds of disasters happen I realize how little I know.

I understand that most of TX is on its own grid and that is somehow making everything harder. But what, physically, is the electrical grid? And what does it mean that the utility providers couldn’t turn the power back on due to increasing instability? I read the ERCOT guy said, “I know this has been a terrible problem, but we don't want a blackout that could last for a very, very long time where the system has to be rebuilt.” What would need rebuilt and why?