Basically they're saying it's their way or the highway, except their foolishness or you're the enemy. It's pathetic....ridiculous, if someone says it to you personally....cuff'em around the head.
Wikipedia definition:
You're either with us, or against us is, technically, a
false dilemma logical fallacy. It is commonly used in order to polarize situations and force the audience to either become allies or to accept the consequences of being an enemy.
"You are either with us or you are against us in the fight against terror" - George.W. Bush
"You're Either With Us or You're With the Terrorists" - Vladmir I. Lenin