[wmirkru]

What does Wajib mean?

As Shakkibat said, and i'll add that in terms of them pertaining to salaah, missing a fardh out invalidates the salaah [wether read individually or in jamaat] and missing a wajib neccessitates for two extra compensatory sajdahs done after the last atahiyat; if a person fails to do those two extra sajdahs and finishes the salaah, then salaah becomes disrputed to the extent where it should be performed again, allthough the obligations of salaah would be considered as offered

so basically something being 'wajib' means it has to be done, but since there is a valid opinion that the order of surahs dont have to be followed [i.e, it is reccomended but not wajib] then you can adopt this opinion thus your salaah will still be ok if the reccomended order is not strictly adhered to