Positives are used to expose an image onto your screen once it has been coated with a layer of emulsion. They can be hand-made or digitally printed, but it's very important that you get them right. If you have a problem with your positive, then you will have a problem when you expose your screen and, no matter how good a printer you are, if your screen is exposed incorrectly, you won't be able to fix it.
The two main rules with a positive are that it is on a transparent paper, which will allow light to pass through it, and that the image you want to apply to your screen is dark enough so that it will block out the UV light during the exposure process. If your positive is a material that doesn't let light through, then you will be exposing a shape the size of the paper onto the screen. Similarly, if the image on your positive isn't dark enough and the light can get through, then nothing will be exposed.
With digitally printed positives it's important to make sure that they have been separated properly beforehand. All the artwork must be pure black. This can then be printed out on tracing paper, acetate, or transparent polyester. Tracing paper is the cheapest, but it doesn't have that much longevity. If, for example, liquids touch it then it can be ruined very easily.