The solution is an alkali.
Usually with the pH value range of 14, substances with pH 7 can be called neutral. Meanwhile substances lower than pH 7 are acids, the lower the pH is, the more acidic it is. Such as cola, it has a pH 2, which is very acidic.
In opposite, the substances with pH over 7 are called alkalis. Again, the larger the pH value is, the more alkaline it is. So pH 13 is a strong alkaline therefore it it corrosive and can clean the toilet well.