For professional printing, bleed extends the printable area beyond the page edge to allow artwork to be trimmed. You can set bleed distances, display bleed guides (so you can more easily design into the bleed area) and then include bleed at export time.
To allow for inaccuracies in the trimming process in professional printing, it's a good idea to extend these elements beyond the 'trim edge' (TrimBox), i.e. the page dimensions defined in Document Setup.
Bleed guides are a visual aid only. They assist you in positioning 'bleed' elements (typically images) that you want to run to the edge of a trimmed page. With bleed guides switched on, you can display the printable area extended by the Bleed distance(s) you specify.
The Include bleed setting in the Export dialog includes the bleed area in the actual output page size.
For single pages, the terms 'Left' and 'Right' bleed are used, while for facing-page spreads the terms 'Inner' and 'Outer' are used. For the latter, Inner bleed is only relevant for first and last single pages or when exporting (printing) as single pages (rather than spreads).
The guides are shown as a red outline that extends beyond the page edges by the chosen Bleed values.
The Snap to spread option snaps to page edge and bleed equally.
Do one of the following: