Ramp-up Period defines the time period where all user must be started. For instance, with 5 users and 5 second Ramp-Up Period, we get a delay of 1 second between the start of each user (5 users / 5 seconds = 1 user per second). A value to 0 means that JMeter will start immediately all users.
Loop Count defines the number of times that the test plan must repeat.