Screen printing is a technique widely used to imprint art onto t shirts and other garments. Ink is pushed through a woven mesh that has had artwork stenciled onto it--the stencil blocks all parts of the screen except where the artwork is, and therefore ink only gets onto the shirt in those places.
Because of the nature of pushing ink through a screen, thin lines and small details are difficult to render. When putting together art to be printed onto a T shirt, large blocks of colour and thick lines are recommended.