There are two types of lexiles, the lexile reader measure and the lexile text measure.
Reader Measure: This number can be found via a reading test or program. "For example, if a student receives an 880L on her end-of-grade reading test, she is an 880 Lexile reader. Higher Lexile measures represent a higher level of reading ability. A Lexile reader measure can range from below 200L for beginning readers to above 1600L for advanced readers. Readers who score at or below 0L receive a BR for Beginning Reader."
Text Measure: This number can be found when a book, article, or text is analyzed by MetaMetrics. "For example, the first "Harry Potter" book measures 880L, so it's called an 880 Lexile book. A Lexile text measure is based on two strong predictors of how difficult a text is to comprehend: word frequency and sentence length. Many other factors affect the relationship between a reader and a book, including its content, the age and interests of the reader, and the design of the actual book. The Lexile text measure is a good starting point in the book-selection process, with these other factors then being considered. Lexile text measures are rounded to the nearest 10L. Text measures at or below 0L are reported as BR for Beginning Reader."
"The idea behind The Lexile Framework for Reading is simple: if we know how well a student can read and how hard a specific book is to comprehend, we can predict how well that student will likely understand the book."
Lexiles can assist you, as the educator, in locating books for your students which are within or near their reading level. Lexiles can also assist readers in allowing them to find books which are at their reading level or will challenge them. Furthermore, the Common Core standards endorses the use of lexiles, the language arts standards can be found HERE.
One disclaimer, lexiles do not account for content, therefore reading the summary of the book is essential for high level readers in young grades. The content must also match the learner. (S)he may not be ready to read about the content within a particular higher-level lexile.