Crushed red pepper flakes are made from hot dried red peppers, normally cayenne, which have been crushed. Frequently there is a high percentage of seeds as well making for a very hot addition to foods. Used in cooking or as a condiment with various foods including to top pizza.
You can purchase red pepper flakes in most grocery stores in the spice section.
You can make your own if you have some small red, dried chile peppers. While cayenne is typically used you may also use de arbol chiles or Thai red chiles. These varieties are all very hot. Use whole chiles with the stem removed but don't remove the seeds or veins. Place the chiles in a small, clean, coffee mill, and pulse until they are the size and texture you like.
Store the red pepper flakes in a spice jar or tin. The flakes retain their potency for a very long time.