Use the adverb within when you need to say that something is on the inside. If you find that the door to your brother’s room is locked from within, that means it’s locked from the inside and he really doesn't want you to enter.
Within often refers to something on the inside of an object or structure — like those screams coming from within the haunted house. You can also use the word more figuratively to describe something that’s in your soul or mind — something that’s internal. For example, you might feel a fire within you as you start getting excited about playing in the big game.