Normally when you upgrade an affix the roll within the tier is randomised. So if you have a helmet with a T3 health it’ll give somewhere between 26 to 40 health, and when you upgrade it to T4 it’ll give somewhere between 41 to 55 health. Normally the value it had within in the range at T3, whether it was at the top, bottom, or middle, has no effect on the value it will have within the range at T4.
The glyph of order changes that so that its position within the range stays the same, i.e. if it gave 40 health at T3 it’ll give 55 health at T4. This can be used to ensure that you get good rolls on your T5 affixes.