A Pre-Processor element is executed just before the request made by the sampler. If a Pre-processor is attached to a sampler element then it will execute just prior to that sampler element running.
A Pre-processor element is used to modify the settings of a sample request just before it runs, or to update variables that are not extracted from response text.
Following is a list of all Pre-processor elements provided by JMeter:
The following image shows how you add a Pre-processor element to your test plan.