There are four major guidelines to consider when making a page:
Purple Margin/ Safety Zone: The purple line is the margin line. All important text and images should be inside this line. This ensures there is no chance they will be cut off during production. It also creates a nice padding around the edge of the page, making it look more visually appealing.
Black Cut Line: The black line in between the margin and red bleed line is the cut line. This is where the book pages will be cut during final production.
Red Bleed Line: The outermost red line is the bleed line. The bleed line is where all of your backgrounds should be pulled out to. This is to avoid white lines on the edges of your pages due to the small margin of error that exists when the pages are cut.
Gutter/ Middle of the Spread: The gutter is the very center of the spread, or where the two pages are bound together. Content gets lost inside the gutter, so make sure to never put text across the gutter, unless you split it up to rest inside the two purple lines.