What it does
Intimidate is an evasion ability that means a creature can only be blocked by artifact creatures and/or creatures that share a color with it. So a black creature with intimidate can only be blocked by black creatures or artifact creatures; a colorless (non-artifact) creature with intimidate essentially can't be blocked by anything except artifact creatures.
The most common mistake is forgetting that artifact creatures can always block intimidate creatures regardless of color, and that multicolored creatures only need to share one color to block. Color comes from the actual creature, not just its mana cost, so anything making your blockers the right color (or making the attacker colorless) changes the math.
In Commander, intimidate is weaker than it looks because the format is full of multicolor decks and artifact creatures across many tables, so a single-color intimidate creature is often easily blocked. It's a deprecated keyword, largely replaced by menace.
