This polite gesture allows you to thank all of the people who helped you with the project, without falling under the category of citations.
For example, a landowner may have given you permission to take samples on their land, or one of the computer science departments may have helped you to recover most of the dissertation that you somehow forgot to back up before the virus hit!
Either way, it is always nice to give them a thank you in a special section, inserted after the appendices and marked as 'acknowledgements.'
This can be as long and as short as you want, but it is not a speech for an Oscar, so there is no need to thank your mother, your agent and your dog.
Sometimes, your supervisor will have had so much input that you can put them as a co-author for the paper. At other times, they should be the first name in the acknowledgements.
In terms of style, some departments keep the acknowledgements strictly formal, with just the name, whilst others encourage a semi-formal approach, with a short note about how the person or department helped you.
Really, it is down to your own preference and it is unlikely that your paper will be downgraded because you used a semi-formal tone.
If it is possible, taking into account confidentiality and restrictions, it is good practice to supply a copy of your work to the interested parties, although you should check with your department that this is permitted.