Layouts are templates that tell Squarespace how to display your site, or in technical terms, they tell Squarespace what HTML to write to the browser. Layouts are made up of two separate parts: HTML and StringTemplate code. Your data (journal entries, pictures, etc) is completely separate from your site's layout, meaning that you can switch between layouts at any point. Your content is what Squarespace truly manages.
Can I increase the width of my layout?
Yes. Enter Style Editing Mode in the upper right corner of your screen. Navigate to the Banner & Navigation tab, and click the button to change widths. Choose the area on your site you'd like to change from the drop-down menu, and use the slider option to adjust this, or manually enter the pixel width into the current value field.
We have an illustrated guide for this here.