A reader is a book that is used for reading exercises.
A level is a rating. It reflects the difficulty to do a task. Here the task is to read.
The level of a reader is determined by various criteria like these:
How many words are in a sentence?
How many words are on a page?
How many words are in the book?
How many different (unique) words are in the book?
Bloom has some levels by default. You can configure them differently or add more levels.
Each level has a Word counts display. You need to review word count information.
Click an item to open an html file with information. This file is installed with Bloom.
Also there may be other useful information on the internet:
https://vimeo.com/channels/bloomlibrary/121689461 is a video that can help you learn about levels.
https://pdf.usaid.gov/pdf_docs/PA00JW1V.pdf
https://www.readinga-z.com/readinga-z-levels/leveling-criteria/