treads in this picture are 1-1/8 inches thick. The stair tread are also
known as the steps which we use to climb up and down the stairs. The
stair treads attach to the stair stringers and go on top of the stair
Some stair treads will be made out of plywood and others out of construction standard lumber such 2x , 3x , 4x building stock. If you were building an exterior set of stairs you will probably be using 2 x 6 or 2 x 12 for your stair tread. If you do end up using plywood on the exterior and it isn't sealed properly, by either painting or using an epoxy coat finishing system the plywood will end up getting wet and delaminating or coming apart.
Tip keep in mind when building a set of stairs like this it's helpful to glue the stair components together to eliminate as much squeaking as possible. The stair treads in this picture were glued to the stair stringers but it's also helpful to glue the back of the stair tread to the riser and the top of the riser to the bottom of the stair to tread, for additional strength.