I guess it means that once you have earned (or deposited) the required amount you should maintain this (without making another deposit or widrawal) for no less than 28 days.
“The end of the 28 day period must not be more than 1 month before the date of your application, for which you must show bank statements/other evidence.”
At this point you only have barely 2 days to lodge your visa application. Otherwise your bank documents will exceed 1 month and the embassy will not accept this anymore as a valid document and your visa application will be refused on these grounds. So you should plan ahead before lodging your application.
