"In between" indicates that something is placed after some thing A and before some thing B. For instance, in the word "album", the letter L is between the letter A and the letter B. It is normally used with physical objects. For instance, in a sandwich, the meat is placed between two slices of bread. It can also be used with locations. We can say that something is "between here and there", meaning that you will encounter that thing somewhere along the way from "here" to "there". Note that "between" and "in between" mean the same thing, unless you use "between" to compare two things, which you cannot do with "in between". The "in" in "in between" is non-standard and superfluous, but it is commonly heard.

It's pretty cool today. I'd say the temperature must be somewhere in between 10° and 15°C. Let me look at the thermometer. Yes, it's 12°. Orange is a color that's somewhere in between red and yellow. The English alphabet starts with A, ends with Z, and has 24 letters in between.