They are the same. Essentials Unit 5 = Essentials Lesson 5.
In early 2020, we changed the title of the divisions of instruction within Essentials from Lessons to Units. Each Essentials book will have the wording updated as it is reprinted.
This wording update reflects the fact that each one includes multiple sections, covers numerous different language arts skills, and is designed to be completed over multiple days or even weeks. We found that the term "Lesson" led some people to assume the content had to be completed in a certain amount of time, and our hope is that updating the title to Units will encourage greater flexibility in scheduling so that teachers will feel free to adapt the pace to the needs of each student or class.