Idol is written as "アイドル" in Japanese, which can also be read as "idle".
Idol is also referred as "unit" while on the field.
Idol can be placed in both "Idol Area" and "Battle Area". It can also move between those zones during Cast Phase.
You can Check an idol card in your Idle Area to pay costs (works similar to casters and burst cards).
At the beginning of your Cast Phase, if you have an idol on play that you have called from previous turns, you may pay maintenance cost (located at the top left side of the card, above log-cost). If you don't, the idol is put into Check state.