A step drill can be used to drill a range of different sized holes with a single bit. Because each step size drills holes about 6.5mm deep it is primarily used for drilling thin material such as sheet metal up to 6.5mm

They don't have a twist like a normal metal drill bit, so they are ideal for drilling through sheet metal and other thin, rigid materials. 

They shave off material slowly and make progressively larger holes as you go from step to step. They also make a nice, chamfered edge to the hole.

PTI HSS Tin Coated Step Drill 6.0 - 30mm in 2mm increments

• Metric Self-Starting Multiple Hole
• Sizes 6.0, 8.0, 10.0, 12.0,14.0, 16.0, 18.0, 20.0, 22.0, 24.0, 26.0, 28.0, 30.0
• HSS Tin coated
• 1/4” Hex Drive Shank