As an example local_reminders implements this function even if it doesn't do anything with it:
function xmldb_local_reminders_install() {
// What should be done when the plugin is installed for the first time.
}So in the function name xmldb_local_reminders_install(): local is the plugin type (it's in the local subfolder in the Moodle source code folder) and reminders is the plugin directory (the plugin source code is in the local/reminders subfolder).
There's also a description in the dev docs.
If you're still stuck, tell us the name of the plugin as it appears in $plugin->component in version.php.