/shorten is a new command and feature that was released with Midjourney V5.2. When you use it, Midjourney will recommend shorter prompts but more importantly show you how strong or weak every token in your prompt is, assigning them ratings from 0-1.
This is a great way to compare tokens.
The scoring isn't perfect: sometimes tokens with very low scores have an outsized impact on your results.
For now, it should be used to check which words are the most effective and which are not as effective. Keep in mind that even if words "aren't effective" they can still have a visible impact on your result.
Let's use this prompt as an example:
stars and galaxies illustrated, simple, featured on pixiv, muted colors with minimalism, irina nordsol kuzmina, a hazy memory I'll type /shorten then I'll input my prompt:
You can see the Midjourney Bot bolds the important tokens and crosses out the tokens that don't matter:
The most useful thing to do is click on the Show Details button on the bottom:
You will see a visualization of which tokens are weighed the most heavily. This is always fascinating to see!
The tokens weighed the most heavily were "pixiv (1.00)" and "irina nordsol kuzmina (0.91)".
Here are some tests of the Option buttons, which strip out more and more of the original prompt. Keep in mind they will strip out parameters:
stars and galaxies illustrated, simple, featured on pixiv, muted colors with minimalism, irina nordsol kuzmina, a hazy memory 
stars and galaxies illustrated, pixiv, muted, irina nordsol kuzmina
stars and galaxies illustrated, pixiv, muted, irina nordsol kuzmina
stars and galaxies illustrated, pixiv, muted, irina nordsol kuzminaThe general aesthetic is preserved, however the actual aesthetic I wanted required the "ineffective words".
Let's do the same test with some very long prompts:
The biggest jump is between the last two: after 2 bolded tokens "masamune shirow (0.01)" and "concept (0.02)" are taken away the generation becomes realistic.
Looking Out the Window of an Apartment in a Big City, Epic composition, Wide angle, by Miyazaki, Nausicaa Ghibli, row iceland, JRPG setting, perspective concept art, lively street scenes, masamune shirow, renaissance, tsutomu nihei, tekkonkinkreet
Looking Out the Window of an Apartment in a Big City, Epic composition, Wide, Nausicaa Ghibli, iceland, concept, lively street, masamune shirow, tekkonkinkreet
Looking Out the Window of an Apartment in a Big City, Epic, Nausicaa Ghibli, iceland, concept, street, masamune shirow
Looking Out the Window of an Apartment in a Big City, Nausicaa Ghibli, icelandThe biggest jump is between the last two: after 2 bolded tokens "masamune shirow (0.01)" and "concept (0.02)" are taken away the generation becomes realistic.
