Near the bottom And this whole guide
Basically it is taking an item and upgrading the defence/stats usually resulting in higher stat requirements.
For example, my sorceress' up'd Viscerataunt has 245def, 72%chance to block but requires str 77 and level 51. Another standard Vicey in my stash has 102 def 60% chance to block, str 38 level 28.
For more details, read the Faqs; but that was a reasonable example.