Something that has impurity is not pure. For example, water that is clean would be called pure water. Water that is dirty would be called impure water. This may result in a change in melting point or boiling point.
Impurities may be harmful. For example, an engine works bad when its fuel is not pure.
Impure materials are useful sometimes. Alloys are metals with impurities. Solar cells are made of silicon crystals, but a very small amount of impurity (0.001% to 0.01%) is needed in them to work.