It's a term bandied about as a joke, used to describe the seeming inability of others to hear us speaking. Often, these "others" are our nearest and dearest. Children. Partners. Students.
We refer to it as "Selective" hearing because it does, indeed, seem to be sporadic. Sometimes they hear us. Sometimes they don't. If we ask our teenager to take out the garbage, they don't hear us. If we ask them whether to buy dill pickle chips or sour cream and onion, they are quick to reply, "Both!". Interesting phenomenon, eh?