DTF stands for Direct-to-Film. Your design is first printed onto a thin PET film and then applied omto your garment using a heat press; the printed film is known as a transfer. Over the past five years or so, it has become the standard for small and medium-sized print runs in the world, primarily because there are no setup costs for individual colors.
How a transfer is made
First the printer applies your colors with CMYK ink and immediately prints a solid layer of white over them. That white layer actually does the trick. Without it the colors of your print would shouw through. A bright yellow print on a black shirt would turn into a pale olive green.
Then the wet film moves through a powder unit. A fine hot-melt powder gets distributed onto the sheet and sticks only to the wet ink. The rest is vacuumed away. Then the oven melts the powder into a layer of adhesive covering the whole print. The final product is a dry sheet that can be stored for months before using it.
So what does this mean in real-life?
For you, it comes down to three things. You don't pay extra for each color, because the printer does everything in one go. There's no minimum quantity, as there are no setup costs. You only pay for the surface you actually use, so a small chest logo is significantly cheaper than a large back print.