The phrase "extra mattress" refers to an additional mattress that is not typically used or needed. It usually implies having a backup or spare mattress available, beyond the regular or necessary ones. Full definition
There is enough space for an additional extra mattress on the floor of bedroom 2 in case of 3 children. (gili-paradise.com)
All rooms have a rustic cabin feel and seem to have been designed for Thai families, with extra mattresses coming standard in both the low - and mid-range rooms. (travelfish.org)