Mentsuyu is soy-sauce-based liquid added mirin, sake, and sugar, etc.  Mentsuyu is mainly used as soup for noodles, as guessing from the meaning of mentsuyu in Japanese.

There are many kinds of mentsuyu such as concentrated and non-concentrated type. There are also many kinds of dashi used.

Nowadays, we use mentsuyu for various dishes as a liquid condiment like soy sauce with dashi.

You can learn a recipe for homemade mentsuyu at this page.