
4 answers
The Salesforce help and training is a great resource. Here is the article about Field Update Actions.
Essentially, a Field Update is one of several actions that can happen when a workflow rule is evaluated to True. Workflow Rules are evaluated every time you create and/or save a record, and can kick off automation such as Field Update, Email Alerts, and other things.
If you go in to Setup and look at your existing Field Updates, you should find one that is updating this field, assuming the note you found is true. Then, you can see the Workflow Rule that is triggering the update, and find the details of that rule.