Privacy levels of a data source are a good way to increment our reports’ security as they specify the isolation level that defines the degree that one data source will be isolated from other data sources, in other words, how the different sources restrict their information to certain users or does not restrict, apart from how the sources interact with each other by being folded following a hierarchy. Although a restrictive isolation level blocks information from being exchanged between data sources, it may reduce functionality and impact performance.
DIFFERENT PRIVACY LEVELS
The different options to set the privacy level of a data source in PowerBI are None, Public, Organizational and Private levels, each one of them with their own characteristics and contributions to security. From this point we will go through every level explaining the benefits and the case of use.

A None privacylevel data source does not have any privacy which is a bad practice and should be avoided, for cases where no privacy is required, a good practice is to set the level to Public instead of None.
A Public data source gives everyone visibility to the data contained in the data source. Only files, internet data sources, or workbook data can be marked Public, and its data may be freely folded to other any other source.
An Organizational data source limits the visibility of a data source to a trusted group of people. Data from an Organizational data source will not be folded to Public sources, but might be to other Organizational sources, as well as to Private sources.
A Private data source contains sensitive or confidential information, and the visibility of the source may be restricted to authorized only users. Moreover, data from a private data source will not be folded to other sources, not even to other private sources.
To sum up and give some good practices, it is highly recommended to avoid None level and set the privacy to Public instead, which gives data visibility to every user. It should be configured a Private level only to those sources with sensitive information to avoid leaks or any unwanted changes, but it is not recommended to set every data source that requires any level of visibility restriction to Private, it is better to configure it to Organizational and include the people in which you are interested in having data visibility to the group you trust.
CONFIGURING PRIVACY LEVELS
You can configure the privacy level of the data sources of the report from PowerBI Desktop or Power Query, not from PowerBI Service. If you are in PowerBI Desktop, open the ‘File’ menu on the top left corner, go to ‘Options and settings’ and then select the ‘Data source settings’ option.

While in the case of Power Query editor, look for and click on the ‘Data source settings’ option on the Home ribbon.

Once you get to the Data source settings, a new panel will appear with a list of all the data sources that are being used in the current report.

To configure the Privacy level of the data source you are interested in, select it and click on ‘Edit Permissions’ and another panel will open up.

In this menu you only need to select the Privacy Level you want from the dropdown marked in red and click on the OK button. Then just close the Data source settings panel and you have changed the Privacy Level of a data source!