EI = exposure index, an abritrary "re-rating" of the film to suit the contrast values of a scene or your prior experience (ideally) with that particular type of scene's contrast.
Typically EI references a different film speed to provide over-or under-exposure. An example is Velvia (transparency film with a bad temper! ) rated at EI40 is +0.3 (third-stop) over-exposure; EI32 is +0.6 (often too much!). It's never referred to as ISO40 or ISO32. Negative films have a lot more exposure than slide films, and so an EI is often given in the range of one or two stops above or below, then development adjusted to suit. EI has more use in LF where sheet film allows individual development to a very fine degree. With roll film (120, 35mm) you are simply giving more or less exposure to suit and possibly even then specifying push or pull processing (something else for you to learn ... later!)
The best way to learn about exposure index is to load some film up, re-rate it (e.g ISO 100 film to EI 200, maybe other indices), expose it normally but importantly, keep notes as you go along so you can reference them examining the negs/trannies.