Молоток is a slang and jokey word for молодец, which is a noun in Russian corresponding to the English expression "well done". (Reference: Большой толковый словарь под ред. Кузнецова).
The reason exactly the word молоток is used is not explained in dictionaries I saw. But one could speculate with a rather high chance of hitting the point that
- the two words sound similar due to sharing the first two syllables and the same stress on the last syllable
- the word молоток literally means hammer i.e. an instrument used to accomplish a task requiring some skill.
Молодец
etc