Hi!
Idle Animation is a Term coming from Game Design, it's the animation/animations that loops when the Player doesn't press any buttons, so the character doesn't turn into a stiff statue that doesn't move at all, which would break the immersion into the game. If an idle animation is done very well, they can also flesh out the character with a bit of visual story telling.
Idle animations are mostly stuff like, lightly bopping up and down or back to front, blinking every once in a while, scratching themselves or inspecting gear.
There are also more worked out idle animations. A famous example would be Sonic, who starts doing flips and trick the longer he stands still, before ultimatively killing himself because he is so bored.
In the realm of 3D Animation, idle animation is mostly used for background characters in standing crowds, having them blink, slightly shift weight and having them look around and their eyes dart. It's often multiple generic and easy animations that are fed into the crowd engine and played at random on random characters.
I hope that explains it
