Your jittered time is:
Enter your base time and a jitter period; this tool will add a random amount of time (not more than the jitter period) to the base time.
The idea is that when you're scheduling an automated task (e.g. scraping a web-page, taking a backup, etc.) you might want to perform the task every hour, but if you schedule it exactly on the hour, you're unintentionally coordinating with other folks who might also be doing that task, which could overwhelm any systems you might be interacting with. Adding a bit of genuine randomness prevents that.