Syncing BOMA with Xero HQ gives you access to the Xero content in the BOMA Content Library and ensures you always have an up-to-date contact list for your email campaigns.
Your Xero HQ login contains an email address and password. If you don't have Xero HQ access, the login can be entered by the person in your organisation who does, and it only needs to be entered into BOMA once. Your Xero HQ login is encrypted and will not be visible on BOMA.
Further Reading:
How do I Import Contacts from Xero HQ
How do I sync my Xero HQ contacts?
How do I import my clients from Xero HQ?
How regularly does BOMA sync with Xero HQ?
How secure is my Xero Data when I sync BOMA and my Xero HQ account?