Export for actionscript

People - a very basic question for you. I’m making a loads of buttons and as I go along I’m curious and thought I’d ask…

What does “export for actionscript” actually mean when you create a button, and is it necessary for me to click it?

Also - is there a point in selecting “export in first frame”, and would this slow down the loading of my swf, or slow it down?

Just want to make sure I select the right choices - anyone can help?