I haven't used templates that I haven't created myself, so I can't say from experience that it isn't what I think it is…but that being said, I think it's a text frame that holds a missing font. When you have a document that uses a font that you don't have on the machine you are using at the time, InDesign's way of telling you the font is missing is to highlight the text in pink. If you go to Type>Find Font, you will see a warning symbol (yellow triangle with exclamation mark) next to any missing fonts. You can use that same window to replace the font with something you have. Or, you could try to find the font that's missing and activate it.
Those are your margin guides.
You can show and hide guides using the View>Grides & Guides>Hide Guides
Best to stick within the margins though - or talk to your printer about the best practice for the guides.
You don't want text too close to the edge, as it has potential to be trimmed when cutting, resulting half a letter being shown...eek.
Solid color, or just an outline? If it's an outline, it's the margin guides that correspond to the margin settings you chose in document setup. To make that disappear entirely, choose margins of 0, but it's not a bad idea to use a value around .125"/3mm to show you a safety zone for cutter misalignment.