Travertine is a type of limestone formed inside hot mineral springs. During this formation process, gasses escaping mud and calcite leave behind holes in the limestone. These holes can range in size from invisible to the naked eye up to holes large enough to put a fist through.
The holes in travertine are in no way a defect in the stone, and in fact give it some of its beauty. To help the stone hold its integrity, these holes should be filled either at the factory or during installation.