Soylent is a soy-based food replacement drink. It is part of the slowly emerging food simplification movement, which products like Soylent, Mealsquares, and Joylent are pioneering. Soylent, being a liquid meal replacement, may be thought of as a protein shake or a diet shake, but without the extra protein and with a normal amount of carbs and fat.
What then, is the point? Well, the theory is that a certain segment of the population is very busy and worried about eating healthily, but not too concerned with flavor. Theoretically, drinking five Soylents a day gives you everything you need to be healthy -- 25 minerals and vitamins, fiber, fat, carbs, and protein. However, Soylent does not actually recommend eating nothing but Soylent.
Soylent doesn't taste great. It tastes good enough that you don't mind drinking it, but not so good that you are tempted to have any extra. It's hard to describe, but if you try to imagine soy milk optimised for health rather than flavor, you're most of the way there (although it is sweetened). The company is aware that this is not a great selling point, and have flavored versions: Coffiest, Soylent Nectar (fruit flavored -- or, perhaps, Froot Loop flavored by some reviews), and Soylent Cacao. Of these I have only tried Coffiest, and while it is a great way to get some extra caffeine with your Soylent, taste is not its selling point.
I have found it to be a very good way to get a light brunch in at work, and would recommend it to anyone who wants a quick and easy meal replacement but isn't on a diet. I am particularly drawn to Soylent because it is vegan. You may well find other dairy-based drinks that perform the same function and taste better.
If you don't mind an extra 30 seconds of preparation, the cheapest way to get your Soylent fix is to buy Soylent powder. (I add some instant coffee, 'cus caffeine). There is also a Soylent Bar, eight a day to stay well-fed; their availability has been highly variable, and I have not been able to sample them.
If any of this tickles your fancy, you can check them out here.