Can someone help with this?

It means they want you to start work April 11. You wont be training until the sometime around the 25th but you will still be working before that. Their not opening the store until May 20th, but likely they need workers to help set things up or whatever. So really they are just letting you know the situation. Really all you need to know is you will start work the 11th and that is when you will start getting paid.