The noun presumption pretty much means jumping to conclusions. It is taking something for granted — an idea, an answer, an event — without having any real knowledge about it, and that is usually not a good thing.
Making a presumption means assuming something is true or false without getting all the information necessary for verification. You can decipher this from the prefix pre, which means "before," together with the sume — from the Latin sumere, "to take." Because so many times a presumption turns out to be false, the word has a negative connotation to it; there's something reckless about making up your mind before you know all the facts.