Noodle soup base can be made easily from scratch using soy sauce, mirin, katsuobushi and kombu. But for the people who are busy, there are ready made noodle soup base sold at stores. Actually, many Japanese people use store bought ones.
Noodle soup base is concentrated liquid, and pretty convenient. Many Japanese cooking flavorings are based on the same combination of dashi, soy sauce, and mirin. Depending on how much water to add, the soup base can be udon noodle broth, soba noodle broth, dipping sauce for tempura, oden or liquid to stew vegetables. If you add sugar, it will be sukiyaki sauce.
I use noodle soup base to make a quick Japanese style salad dressing as well. Just combine noodle soup base, vinegar, and sesame oil. It tastes pretty good.
The strength of the soup base varies, so always look for the instruction on the bottle.
Noodle soup base can be found at Asian grocery stores, but if you prefer to order online, here are some links;