Qualifying period refers to the time duration for which your accumulated Loyalty Points will account for your club status.
For Example: Diamond Club has a qualifying period of 1 month. This means that you can upgrade to Diamond Club by accumulating 1000 Loyalty Points in one month. Let's say you are a Silver Club player at the beginning of March 2019. You can upgrade to Diamond club by earning 1000 Loyalty Points in the month of March 2019 only.