sidebars and widget areas

Can you tell me what is the difference between the Main Sidebar and the First Front page widget features? I noticed the description of the Front page feature was if you were using an optional front page template with page set as static. Is this if you did not have to create a home page? .I did have to create my home page and set it to static .Otherwise I could not set my default home page to static – it was no available to set to static until I went into the Menu and deleted home and re-created it.
However, I could get the Image (jetpack) widget to display the caption (on my home page in the sidebar)when I chose the Main Sidebar, but not the image. Does the image have to be sized to fit in the sidebar? Maybe my image was to large?
I am using the Twenty Twelve theme.
Thank you.