A floating signifier is a word or phrase that has no clear, agreed on meaning, but feels like it means something. The idea of floating signifiers was developed by the anthropologist Claude Lévi-Strauss, who used it when critiquing the idea of 'mana' as unifying force in human spirituality, which he attacked as essentially meaningless. It has since been used in other contexts, although it is primarily used in the field of semiotics.
In political contexts a floating signifier gives people a way to build a feeling of community or unity without having to worry about the specifics. While it can be difficult to tell the difference between a floating signifier and a political dog whistle (and it may be possible for a signifier to act in both capacities), things like American, patriot, freedom, Hope, and Strong and Stable can pretty much refer to any values you might personally hold, freeing politicians from saying things that voters might disagree with.
In marketing the phrase has drifted a bit. It is not uncommon to hear the 'floating signifier effect' used to describe brand building, as when the diamond industry rebranded to make diamonds the central symbol of undying love. In this case the signifier (the diamond) is very clear and is intended to give the same meaning (romance) to every person that hears it, however the meaning has 'floated' from where it would rest naturally to where the diamond producers wanted it.