Japanese Tsuyu つゆ is a sauce which is mostly used as a soup base in noodles dishes such as soba, somen, and udon. It is usually made with dashi (made with dried bonito flakes, konbu seaweed, or mushrooms) and soy sauce.
It is also referred to as “Men-Tsuyu” 麺つゆ (noodle tsuyu) since majority of its usage is tied with noodles. Mentsuyu could be specified for different types of noodles, i.e. soba, somen, udon, etc. There are two types of mentsuyu, concentrated 濃縮 and straight ストレート. Concentrated ones need to be diluted with water before using, while straight can be used directly as it is.
On lazy days, if you would like to have a bowl of warm Japanese noodles, you can simply pour (dilute) some Tsuyu, add some cooked Japanese noodles, and enjoy!