Content on Hover or Focus applies to tooltips, custom dropdown previews, and any extra content that a hover or keyboard-focus event reveals. That content must be dismissible without moving the pointer (typically with Escape), remain visible while the user’s pointer is over it, and stay visible until the user moves away or dismisses it.
Without this, a tooltip that vanishes the instant the mouse shifts a pixel is unusable for anyone who needs to read it slowly, including screen magnifier users and people with motor impairments who cannot hold a pointer perfectly still.