it's a layer of ice on the ground that is virtually clear and makes it nearly undetectable from normal pavement unless you look very closely at it. it's extremely slick like an ice rink and can be very bad if you drive over it at high speeds or step directly on it. what makes it more dangerous than other ice is, obv, you really can't see it.
this road, for example, is covered in black ice. imagine if you are driving fine and then have a small section of this. it can end badly.
that's why northern drivers often rage when people from the south drive on the snow. even tho the weather conditions are perfect for ice to form and there has been a lot of moisture or runoff from melting snow, they'll just see black pavement and go flying along at 60 MPH.
i learned this lesson the hard way when i was 16. snow plows had been through, salt was down on the road and everything looked clear. i took a corner at only 25-30 MPH, hit a patch of black ice and 360'd my car into a snow bank.
plus, once you hit it and lose control, everyone's instinct is to slam the brakes. this can be very bad, especially if your car doesn't have ABS (which wasn't really prevalent when i was 16).