Intimidate is an ability which limits the amount of creatures that can block creatures with intimidate. In other words, a creature with intimidate can't be blocked except by artifact creatures and/or creatures that share a color with it. This basically means that if a green card has intimidate, then only artifact creatures and green creatures can block that creature. Otherwise, the creature is unblockable.
I attack with my Bellowing Tanglewurm. My opponent has a 4/5 Leatherback Baloth. Although my Bellowing Tanglewurm has intimidate, Leatherback Baloth is green. It therefore shares a color with Bellowing Tanglewurm and can block it. 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 it. Now, suppose they have Wooly Thoctar. Wooly Thoctar is red, green, and white. Although it is red and white, it still shares a color with Bellowing Tanglewurm and can block it.