Glossary
By a steep downhill you mean that the road slopes steeply downwards. If the road slopes 6% or more (if the road rises or falls 6 meters in a hundred meters), there is usually a warning sign in place that shows the slope as a percentage.
Note that warning signs are not always in place, for example on longer steep stretches, it is common that there is no warning sign.
When driving on steep downhill slopes, it is often a good idea to downshift to save on the brakes, also keep in mind that your braking distance will be longer downhill and adapt your driving accordingly.