Portable in this context means that the application can be installed on a memory card or USB memory and moved from device to device. There is no dependency on the host computer. There are no register entries required and it can run without needing any modifications to the host environment.

See Portable Apps for a way to create or find portable programs.

Portable programs are usually Windows based but there can be portable programs for Linux as well. Actually many applications for Linux are portable automatically without doing anything special.