Fear is a static ability which restricts the types of creatures that can block a creature with fear. A creature with fear can't be blocked, except by artifact creatures and by creatures that are black.
Fear is a very old mechanic, and has existed at least since Alpha's Fear, although it did not have a keyword at the time and was simply spelled out in the rules text until Odyssey. Intimidate is a newer version of fear that can appear on cards of any color.
If Nora wishes to block the incoming Severed Legion, she can legally block it with the Sliver Construct, as it's an artifact creature; or the Undead Minotaur, as it is a black creature; but not the Canyon Minotaur, as it neither shares a color with Severed Legion nor is an artifact creature.
- 702.35a. Fear is an evasion ability.
- 702.35b. A creature with fear can't be blocked except by artifact creatures and/or black creatures.
- 702.35c. Multiple instances of fear on the same creature are redundant.