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.
Example
Alex is attacking with an Severed Legion and Nora controls a Sliver Construct, a Canyon Minotaur, and an Undead Minotaur.
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.
Comprehensive Rules
702.35. Fear
- 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.