What does this traffic sign warn you of?

You answered correctly. This warning sign alerts drivers to a risk of skidding. It specifically warns about a slippery road surface, which can happen when the road is wet or dirty. That’s why both the danger of skidding when the roadway is dirty and when it's wet are correct. The sign is not about drivers under the influence of alcohol. Remember this sign to reduce your speed and increase caution on potentially slippery roads.