If you press Option-s on an Apple keyboard, you’ll get “ß”. It looks like the letter “B” or the Greek beta but it’s actually a German sharp S, a ligature of “ss” or “sz”. It was used in other language, but it only survives in German.
Up until the 1800s, a medial s, which looks like ſ, used to be in the alphabet. It’s the letter s found only when it’s inside a word. When it’s found on either end, it is called a terminal s. Even before we got rid of it, most of the world had gotten rid of it by the mid 1700s. The main reason was because it can get confused with the letter f. Notice that ſ either has no line going through or a line going half-way through depending on the font, while f has a line that goes completely through.
Here’s a picture from the U.S. Bill of Right with both a medial s and terminal s.
When you fuse the medial and terminal s together, you form ß.