What does the box starting month on the checkout page mean?

I noticed this in the checkout, what is it?

 

You can choose which month you would like to start your subscription.

If you really don’t want to wait till the next month, you can opt to start in the month you made your purchase.

The cut off date for the current month option is the 20th of the month we are in.

Just check whether you want the current month, or the next month.

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