I have been wanting to know this for a long time. It is when a note stays on while some other notes are being played at the same time, it is kind of like a Pedal Point from research. Here are some examples of them being used below, the type I'm looking for is for calming songs I'm not quite sure on the term for it.
Varieties Of Exile, Beirut, at about 2:10 and about 4:10
Some Minecraft Story Mode Music:. Occurs at many places, like 0:14 - 0:24