What is the difference from being stiff and properly covering yourself?

I am getting into Muy Thai and my coach tells me that i am too stiff when i spar but i am just keeping my hands up so i don't get hit. I notice that i keep my hands a little higher then most cus in all the UFC and Boxing matches i watch the fighters don't keep there hands up the whole time they are usually a little below the chin or a little above the chin i keep mine a little below my eyes. Is it that too high?