Punctuation is an ordinary everyday normal part of life, used in all kinds of writing in many different languages spanning the world. The most common punctuation being characters like periods, exclamation marks, question marks, quotations, and more. These might be sufficient for some people, but one relatively common use-case isn't covered by just one of these options, being a question in an excited manner, expressing confusion or disbelief in the question.
Currently, the most common ways to write an interrobang are as "?!" or "!?", although these aren't the true form of it. The true form of the interrobang is this, "‽", in all it's glory. In languages like Spanish, an upside down interrobang is sometimes used, looking like this "¿¡".
The interrobang was invented in 1965, by Richard Isbell on the American Type Founders typewriter, appearing in a scarce few pieces of media such as dictionary's and magazines. Although probably obvious to many people, the interrobang is a ligature of the exclamation point, and the question mark.
The real reason it's called the interrobang is linked to some alternate names of the punctuation it is composed of. Firstly, a question mark can also be known as an interrogative point, giving the prefix interro-. Secondly, an exclamation point, in the context of programming, can be referred to as a "bang", giving the suffix -bang. Putting these together, you get the word interrobang.
The main reason the interrobang fell out of use, although boring, is simply because there wasn't a good enough use for it. At it's core, the interrobang is simply a superfluous, redundant word that died before it could truly live.
Today, the interrobang is mostly a dead symbol, used only by weirdos and nerds with too much time on their hands writing essays about stupid stuff online. Sadly, it will almost always be more convenient, faster, and more understandable to most people to simply write "!?" instead of "‽", barring nerds like the person who wrote this essay.