A few years ago the W3C (World Wide Web Consortium) created a mechanism for browsers to inform web servers that they would not like to be tracked. This is accomplished by including a DNT (Do Not Track) header with web requests. This header is enabled by default in Privacy Browser, although if desired it can be disabled in the settings.
The DNT header doesn't really provide much privacy because most web servers ignore it. For example, Yahoo, Google, Microsoft, and Facebook all ignore at least some DNT headers.
Privacy Browser Free includes a banner advertisement across the bottom of the screen that is populated by Google's AdMob network. By default, Google provides the advertising ID of the device to the ads displayed through this network. This allows advertising companies to build a profile of the device that show which apps are installed (that display ads), how often they are used, and which ads the user is interested in.
Users can choose to disable the advertising ID in Settings, Google, Ads.
The purpose of the free version of Privacy Browser is to allow people to test the features of the app. The standard version can be purchased on the major app stores or downloaded for free from F-Droid.
Verizon, one of the major mobile carriers in the United States, adds a unique tracking header to all HTTP traffic on their network. The Electronic Frontier Foundation has written about the privacy implications of this practice. Due to public pressure Verizon has created a way to opt out of this tracking.