While shopping for a screen printer, you may have come across the term “underbase.” To put it simply, an underbase is a first layer of ink printed onto a garment (usually a dark colored garment) in order for all of the other colors to be printed on top and stand out brightly.
For example, if a bright red was printed onto a dark garment (in this example, a black shirt), it ink would look like a brick red due to how the ink sits on the shirt. However, if another layer of red is printed on it, the color would be bright and stand out on top of the first layer that was printed.
Having no underbase allows you to see some of the fabric texture through the ink, while having an underbase gives the image a bright, crisp look. Below is an example.
With all of that being said, if you are looking for a grungy/distressed look, I would suggest going without an underbase where possible. But if you want crisp, bright images, go with an underbase (it will be worth it).