To play "air guitar" you move to the music as if you are playing an imaginary guitar, maybe like a famous rock musician.

VOA: special.2009.08.14

I don't know how well you can make it out but there's a little broom guy there with glasses, playing air guitar with his broom, and I guess this is meant to capture the spirit of the contributors.

我不知道你们能理解到什么程度,但是在那儿有一个戴着眼镜的男孩，用他的扫帚在弹着吉他，我猜这个的寓意是指要抓住那些发起人的精神内涵。

耶鲁公开课 - 现代诗歌课程节选