Okay kiddo, so a lustrum is a fancy word that means a period of time, just like how you have a week or a year. But the lustrum lasts for a really long time - 5 whole years! That's why it has the word 'lust' in it, because it's such a long time that you really have to want it!
Back in ancient Rome, the people were really into counting time and keeping track of important events. So they would have special ceremonies called 'lustrations' every 5 years to clean and purify things like their buildings and their soldiers. It was like a big party where everybody would come together, and they thought it would make everything better and more lucky for the next 5 years.
Even though we don't do lustrations anymore, some organizations and groups still use the word lustrum to talk about a special time period that marks something important or memorable. So now, when you hear someone say 'lustrum,' you'll know they're talking about 5 big years full of special events and celebrations!