Dashi is Japanese soup stock and the basis for many dishes in Japanese cuisine. It's made with dried bonito and kelp, but can also have shiitake and other dried fish.
Dashi (along with soy sauce, miso, and mirin) is one of the four cornerstones of Japanese cuisine. Given the simple, understated nature of many Japanese dishes, good dashi is what sets apart bland salty water from a deeply nuanced miso soup.
While the ingredients used to make dashi varies regionally, the most basic dashi used in most Japanese dishes is made using dashi konbu (kelp for stock) and katsuobushi (dried bonito). The konbu is soaked overnight in cold water to slowly release the flavor and the resulting liquid is brought to a boil before adding freshly shaved katsuobushi, which is then left to steep for 5-10 minutes.
These days, many households use instant dashi granules, which is like the bouillon cube of Japan. Unfortunately, the stock made from these granules tastes nothing like real dashi and it's often full of salt and MSG.
My compromise is to use dashi bags, they're like tea bags and contain whole ingredients such as shaved bonito and kelp without any additives. Like tea, dashi bags vary in quality, which is usually reflected in the price. They can be tough to find in the US unless you have a Japanese grocery store near you, but you can also find them at online Japanese grocery stores like Marukai and Mitsuwa
While the ingredients are always translated, the instructions often aren't. Luckily you can easily figure out the amount of water to use per pack by looking for either "XXX ml" or "XXX cc" (see the examples below). The measurements are given in milliliters or cubic centimeters, which are the same thing. Pyrex measuring cups include marks for milliliters. If you see two measurements given, this is usually for making a concentrated stock (the low amount) versus a regular stock (the larger amount)
As for time, I usually put the dashi pack in a pot with the appropriate amount of water, bring it to a boil over high heat, turn down the heat to low and then let it steep for 5 minutes. Then just remove the bag and you have dashi.
If you plan to use it on a regular basis you can make a lot of it and store it in the fridge in clean water bottles for up to a week.