Dual-licensed software refers to free open source software that is also released by its authors under a proprietary license. This was done sometimes in the past for administrative reasons, for particular customers that preferred proprietary licensing. However, as long as all the software is also available under an open license approved by the Open Source Initiative then it is still FOSS.
The practical problem with dual-licensing is only copyright owners can do it, so unless a company somehow holds the copyright of every single author, it is usually practically impossible to get permission to dual-license. That is why all the popular FOSS applications will always remain FOSS.