Acronym for Free or Open Source Software. FOSS programs are those that have licenses that allow users to freely run the program for any purpose, modify the program as they want, and also to freely distribute copies of either the original version or their own modified version.
One major reason for the growth and use of FOSS technology (including LAMP) is because users have access to the source so it is much easier to fix faults and improve the applications. In combination with the open license, this simplifies the development process for many enterprises and gives them flexibility that simply isn't available within the confines of a proprietary or commercial product.
Learn more about Open Source in Webopedia's "Did You Know...?" section.