Tshirt fabric is knit, not woven, so it stretches in all directions. The stabilizer we are talking about irons on to keep it in place so you can sew it accurately. It is sold at fabric stores, with various brand names and thicknesses, ususlly white or black. For this purpose, you'd want the iron-on kind.
If iron on stabilizer gets separated from the plastic wrapper, you can still tell the glue side because it is shinier. Place the shiny glue side against the back of the t-shirt, and have fun.