Adapters are used to allow you to fly motors of various diameters in your rockets. Let's say you have a 38mm motor mount. You could fly 38mm motors, or you could fly 29mm motors with a 38 -> 29 adapter. This increases the range of motors you can fly in each rocket.
Please note, you cannot use more than 1 adapter at a time. You can only step down 1 motor mount size. These adapters do not retain the motor in the rocket - you'll need to use a seperate motor retainer, which can be found here.