The melting point of a solid substance is the point at which it turns from a solid to a liquid. It is a commonly used physical property to test the purity and identity of a substance. In general, purer substances have smaller melting point ranges while more impure substances have larger melting point ranges. Ideally, a melting point range will be less than half a degree in range for a pure substance. Since the substance in question has a melting point range of 6.2 degrees C, it is not a pure compound.
For part B of your question, it is not possible to identify a substance by the melting point alone. There are many thousands of chemical solids and many different solids will have the same melting point. Melting point analysis is mostly useful for comparing one batch of a compound to another batch of the same compound to make sure that they match.