A recent article on Politico mentions a list of names from the Obama administration that was recently sent to Republican senators:
Acting Director of National Intelligence Richard Grenell on Wednesday sent top Republican senators a list of former senior Obama administration officials who might have been involved in efforts that “unmasked” former national security adviser Michael Flynn — including former Vice President Joe Biden.
What exactly does "unmasking" entail? I'm asking specifically with regard to Lieutenant General Michael Flynn being the object of many unmasking requests made by senior officials in the Obama administration.