The “gutter” is a typographical term referring to an additional margin added to interior pages to compensate for the part of the page made unusable by the binding process. In a facing pages layout, the gutter margin is on the inside of both pages. Books of less than 60 pages don’t normally need to include a gutter, but files with higher page count should consider using a gutter.
For more information, use this table:

Page CountAdd to Interior MarginRecommended Interior MarginRecommended Interior Margin (Full-Bleed)
Less than 600" or 0 mm0.5" or 13 mm0.625" or 16 mm
61 to 1500.125" or 3 mm0.625" or 16 mm0.75" or 19 mm
151 to 4000.5" or 13 mm1" or 25 mm1.125" or 29 mm
400 to 6000.625" or 16 mm1.125" or 29 mm1.25" or 32 mm
Over 6000.75" or 19 mm
1.25" or 32 mm1.375" or 35 mm