The Pre Processor elements are used to modify the sampler requests before their processing(hence the name pre-processor).How to add a Pre Processors in JMeter-
The BeanShell preprocessor is used to perform some operation using beanshell scripting before a sampler request.
The HTML Link Parser is used to extract links from HTML response fetched from server.
The HTTP URL Re-writing modifier can be added at either Thread Group level or Sampler level to fetch a specific sessionID parameter from the response of a request and then use the parameter's value on other requests.
The JSR223 preprocessor is used to perform some operation using JSR223 scripting before a sampler request.
The JDBC PreProcessor is used to execute certain specified SQL queries before a sampler request processing.
The RegEx user parameters are used to extract HTTP parameters from certain request using a regular expression and then passed as request parameter to other sampler requests.
The User parameters are used to specify values for User variables used within Thread Groups.