Intimidate is an ability which restricts the types of creatures that can block the creature with Intimidate. A creature with Intimidate can't be blocked except by artifact creatures and by creatures that share a color with it. This means that if a green creature has Intimidate, then only artifact creatures and green creatures can block that green creature. Otherwise, the creature is unblockable.
I attack with my Bellowing Tanglewurm which has Intimidate. My opponent has a green 4/5 Leatherback Baloth. Because Leatherback Baloth shares a color (green) with Bellowing Tanglewurm, the Baloth can block the Tanglewurm. However, if my opponent has a Shivan Dragon, which is red, it can't block Bellowing Tanglewurm because it doesn't share a color with the Tanglewurm. A Wooly Thoctar - which is red, green, and white - can block Tanglewurm: although it is red and white, the Thoctar still shares a color (green) with Bellowing Tanglewurm and can block it.