what is ramp up period in jmeter

The ramp-up period in JMeter refers to the time it takes for the load testing tool to progressively increase the number of virtual users or threads from the initial value to the maximum value specified in the thread group. During the ramp-up period, JMeter starts the specified number of threads gradually over time until all threads are running. The ramp-up period is specified in seconds, and it tells JMeter how long to take to ramp-up to the full number of threads chosen. For example, if you set the number of threads to 100 and the ramp-up period to 50 seconds, then JMeter will start two threads every second until all 100 threads are running. The ramp-up period needs to be long enough to avoid too large a workload at the start of a test and short enough that the last threads start running before the first ones finish, unless one wants that to happen. By default, the thread group is configured to loop once through its elements.