From Diablo Wiki
Random Properties are bonus modifiers found on crafting plans of the Blacksmith.
Random properties are what keeps crafting interesting, mysterious, and unpredictable in Diablo III. You never know quite what you'll get, which turns item crafting into a bit of a gamble. This follows in the footsteps of in Diablo 2, but it's very unlike the predictable, guaranteed outcome of item crafting in most RPGs.