𝗪𝗵𝗮𝘁 𝗶𝘀 𝗙𝗢𝗦𝗦?
#FOSS means "Free and Open Source software".
𝗪𝗵𝗮𝘁 𝗶𝘀 𝗙/𝗟𝗢𝗦𝗦?
#FLOSS means "Free / Libre and Open Source software".
Both acronyms identify applications that are not closed source and that offer full transparency of their inner workings and #development processes.
Be careful, as these acronyms are used where a rough categorization is needed and their meaning depends on the context where you find them.