A graded reader is a specially designed book intended for language learners. It uses controlled and simplified language to match different proficiency levels.
These readers may be adaptations of existing works, original stories, or factual books, and are graded by levels based on vocabulary and grammar complexity to suit learners' needs.
The purpose is to facilitate reading comprehension and language acquisition by providing material that is accessible yet engaging, often with multiple levels to support gradual progress.
How are the levels graded?
Different publishers use different methodologies. But usually they control the vocabulary (for example, A2 could have only top 1000 lemmas) and the grammar (tenses and the complexity of sentences). Many try to align with CEFR.
Where to find graded readers?
One option is to just search Amazon for graded readers. Many publishers have a pretty strong focus on graded readers. The most famous ones are probably Oxford and Penguin Readers.
Some apps have published their graded readers as well, such as Lingo Champion. Apps like these are designed to basically treat everything as graded reading - users can see how many unknown words there are in the text, they can simplify texts etc.