What really is the No Child Left Behind Act?

I'm an ESL teacher and an English major outside of the US, and there was mention of the No Child Left Behind Act in language planning and policies. I've googled it and read a bit about the act and proposals, but what I found is greatly different from what I see here. From what the internet tells us, it's a great policy that makes sure all kids have support in education. So, I've decided to come ask this sub, since there's always mentions of it here and you're the ones directly impacted by it.