Autocomplete on GAL is a COS and account level feature. Users can disable this feature from their Options, Address Book tab.
When Autocomplete on GAL is enabled, as users type in the address field, suggested addresses that match the text are displayed. Users can select an address from a list rather than type the complete address.
Enabling the Autocomplete on GAL feature adds more load to the LDAP servers. The Zimbra Web client does some caching to lessen the load, but if you are concerned about the potential impact on the LDAP server, you may not want to enable this feature.