Ilvl of and item is the level of the monster that drop that item (or chest)
Is your own level +4 if you imbued that item
And your own level -5/+4 if you gamble/buy that item from and npc (you can't know the exact ilvl without some illegal stuff)
To know the level of the monster, in Nightmare and Hell, the mlvl = Alvl (to know the level of the area, make some fast google search), keep in mind uniques and their minion have 3 more level than the regular monster in the same areas, and champions 2 more level.
In normal their have their own level that don't need to match with the Alvl they are, IMO is a waste of time since you spend very few time in normal, but you can google to see the exact levels of every monster in normal.
And final bosses, and superuniques have their own level that you can find, again, with google (for example, hell baal is lvl99, Hell diablo 94, Hell Nilathak 95, Hell Mephisto 88, Hell Pindle 86..)