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Your pink box shows your margins set in File/Document Set Up.
Edit: this applies for a new document. For an existing document you'll see the margins in Layout/Margins and Columns.
When you create a new document you are presented with a dilaog box that let's you setup the page to your specifications.
Pink box = Margins
Margins are important as this dictates a "safe area" - this is an area far enough away from the edge of the page that during trimming at a printing company that the text or images that do not bleed need to a satisfactory distance from the edge of the sheet of paper that when trimmed it doesn't get trimmed off. Removing vital text or part of an image that you intended to be viewed.
It's also important as if the space is tight on the sheet it can cause stretching when trimmed - and this could inadvertently skew/stretch text or an image/logo when trimmed.
Talk to your printers about good guidelines and safe practices for your margins.
Red box = Bleed
This is very important for any images that are intended to go right to the edge of the page! It's very important that they extend a certain amount usually 3mm some people prefer 5mm and for hardback covers you may need about 20mm - this is something you can agree with with your printers.
If you don't extend an image into the Bleed area then you run the risk of a sliver of white being shown if trimming is not 100% accurate.
And it rarely is 100% accurate. As the printers stack 1000s of sheets of paper on top of each other, and it's mechanically cut. The bleed area is intended to counter for minute shifts in paper and for mechanical errors when being trimmed.
Blue Box = Slug Area
The slug area is a area you can setup on your sheet to include additional information to the prepress department or to the printers themselves.
This appears outside the Bleed area as you don't your information appearing in the bleed where it could potentially end up in printed matter.
You can convey things to the printers like colour builds for logos, any information you think might be relevant - like worried the images are too dark, too light, or any information you want to pass on to the press department.
You will note that I only included 15mm at the "bottom" - this is to make sure that it's outside the bleed area - and I can add in specific information .
If you need to do this contact the printers and ask them which side you should include any slug information.
All these questions should be answered before you start your project. Always consult your printers before you do anything on any project that you're not sure of.