can someone tell me what exactly is msg used for?
Mono sodium glutamate. Think of it as somewhere between salt and sugar. It's flavour is called Umami. Msg is present in many natural products mostly from fish like anchovies.

The reason many restaurants have "no msg" sign is because of the bad press and stigma it has received for health reason such as hyper activity and weight gain. Chinese restaurants have especially received a lot of flack for over using it in their take away food.

For a more informative read I highly suggest checking this out It’s the Umami, Stupid. Why the Truth About MSG is So Easy to Swallow