When a model and photographer (and sometimes a make up artist (MUA) too) work together, either to enhance their technical and creative skills or simply to add to their portfolio.
Similar to a test shoot, TFP (time for photographs) or TFCD (Time for CD) each party provides time in exchange for prints or images. Generally offered by those who are transitioning into a new area of photography or trying to build their portfolio of work but also good for trying out a new style, experimenting with some new equipment or simply helping others out.
This works pretty simple. If I chose you for a TFP shoot, the shoot will not cost you any money, but will be a trade for your time and talent , for the final images. The final images will have my watermark on them. You will get a copy of the images for your portfolio. That Simple.