For professional printing, bleed extends the printable area beyond the page edge to allow artwork to be trimmed. You can set bleed on document setup, display bleed guides (so you can more easily design into the bleed area) and assign colors to them and then include bleed at print or 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 which 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 you specify.
An Include bleed setting in the Export PDF or Print dialog includes the bleed in the actual output page size.
The guides are shown as an outline that extends beyond the page edge by the chosen Bleed value.
The Snap to Spread option snaps to page edge and bleed equally.
Do one of the following: