The ramp-up period tells JMeter the amount of time for creating the total number of threads. The default value is 0. If the ramp-up period is left unspecified, i.e., the ramp-up period is zero, JMeter will create all the threads immediately. If the ramp-up period is set to T seconds, and the total number of threads is N, JMeter will create a thread every T/N seconds.