Heat Transfer Vinyl (HTV) is used to make simi permanent designs that can be ironed on to a piece of fabric. It is very commonly used to make matching shirts for an event of some kind.
These are my first 2 projects both the small scale test (bandana) and large (t-shirt). I ended up doing 6 of the custom shirts using nicknames and year of employment and one bandana for a custom order for a company's Christmas party.