What is a Lexile?
A Lexile measure is a valuable piece of information about either an individual's reading ability or the difficulty of a text, like a book or magazine article. The Lexile measure is shown as a number with an "L" after it — 880L is 880 Lexile. Read more about lexile levels here.
How has the Lexile measure changed with the Common Core State Standards?
MetaMetrics realigned the Lexile bands to support the text complexity that the CCSS require. Working backwards from the level of text students need to be able to handle at the college and career readiness level, they provide a Lexile band for each grade band. These ranges are the level of texts students need to be able to independently engage with as they move through school in order to end up being able to interact with college and career level texts once they graduate. The Lexile bands have been updated to include the new ranges or "stretches" for the Common Core State Standards.
Source: http://legacysupport.nwea.org/sites/www.nwea.org/files/resources/FAQ_Lexile.pdf