A guise is a way that something looks: a way that somebody appears. Now take the word "guise," and add "dis-," which means "off or away," and you get "disguise."
A disguise is something you wear that changes the way you look, so that others don't know it's you. This kid is wearing a disguise! With new glasses and a new nose, plus the crazy eyebrows and moustache, this kid is disguised as a totally different person.
We often say that someone or something is "in disguise," meaning they're wearing something that hides who or what they truly are. Below, Damian is in disguise. Because he's sneaked into an assembly for just girls, he's got to disguise his boyish face and hair.
If something bad or unlucky happens, but something good comes out of it, we sometimes call the bad thing "a blessing in disguise." That means it was really a good thing (or a blessing), but we couldn't tell at first because it seemed to be dressed up as a bad thing. In the book A Farewell to Arms, Henry gets injured, but at the hospital, he gets to spend time with his nurse, Catherine, who becomes the love of his life. You could say that his injury was a blessing in disguise.
"Disguise" is a verb, too. When you disguise something, you change the way it looks so that you can hide it or keep it a secret. For example, if you're bothered by the look of acne on your face, not to worry: you can disguise it with makeup. That means you apply makeup on top of the acne to hide it.
Sometimes, you have to disguise your feelings, or disguise your reactions. For example, if you're laughing when you're not supposed to, then you might disguise your laughter as a cough. Hmm, hmm, hmm—mm, mm. Heh, hmm, hmm. Hrm. Hrm... hrm.