The Acknowledgements part of a project or essay is the space reserved for thanking the people who've helped you along the way, without whom the end result might not have otherwise been possible.
In an academic setting, the Acknowledgements section will usually include the names of supervisors or tutors, and how they shaped the project or guided you in certain directions. It might also include other staff members you've worked with, your editor (if you have one), the students - either fellow students, or people you've taught - who have helped along the way, and companies, libraries, or other organisations that have enabled you to finish your studies.
Outside of the academic setting, Acknowledgements in things like novels and non-fiction works often tend to include family members and friends, but these are usually either left out of educational works or written in the same kind of high register you'd expect with the rest of the section.
Here's a nice little guideline to help spark some ideas for the kinds of people you might like to acknowledge, and how to go about writing it up.
