You're correct in that purging is essentially a deletion of all the data in a QuickBooks Online account. A few things to keep in mind is that a user is unable to perform this action if certain features have been used, such as paying employees with direct deposit. The article that @john-pero provided contains additional insight into this process and I recommend looking over it before initiating this process.
When re-importing from QuickBooks Desktop, all of the data within QuickBooks Online will be overwritten by what's contained in the Desktop file. This action can't be undone and I suggest notating anything that's been done since the import. The Audit Log feature can assist with reviewing activity in an account and I'm including an article for further guidance: View Audit Logs in QuickBooks Online.
Wishing you a wonderful rest of your week. Take care.