It means that whatever strings in your password that are elements in the periodic table, should have atomic numbers (in the periodic table) that add up to 200.
So go find a table on wikipedia, add up the elements it already recognized and choose strategically a couple of new ones to add !
Look up a periodic table and correlate the highlighted letters with numbers on said table.
Change, remove, modify letters until you reach 200 based on the sum of their atomic numbers.
For easy cheese, add H to increment by 1. Repeat capital H until you have reached the sum if you are under. Easy to later add and remove H until you satisfy the condition.