Trackers are a bit like cookies - just a bit of a different flavour.
Well, I’ll try to explain that with the words of a non-techie (@community, in case I am mistaken, pls correct me).
What is a tracker?
A tracker is a bit of code that is shipped within an app allowing to track and analyze the users’ behavior (what he/she is searching, the usage of apps etc). Very roughly spoken, a tracker for an app is what a 3rd Party Cookie is for a website. Certain trackers - as the ones from Facebook - allow to track users even across different apps.
Where do trackers come with?
Trackers are delivered not only with apps from ambiguous sources, they are just delivered with most apps that can be found in Google’s Play Store. Many apps (especially games, dating, fitness and shopping apps) are even stuffed with a variety of trackers and ads.
Examples: Tinder for instance, delivers with its installation currently 13 trackers and Spotify 10. That means: Tinder is sharing personal user data with 13 ‘partners’. Those partners are companies who typically base their business model on the collection, linkage of user data and their commercialization. That’s a huge business. Among those data collectors there are two really important ones, ‘old accquaintances’: Google and Facebook. Google is already well placed as it stores user data from most Android phones on their servers and processes them through their enormously popular apps such as Google Maps, Gmail etc.Additionally, they collect more user data through apps installed on phones. The same with Facebook: Although you may not have an account with Facebook/WhatsApp/Instagram, it is likely that Facebook is holding a collection of data about you - data that originates from installed apps that come with trackers… those are the troyan horses of 21st century…
What to do against?
Thanks to projects such as Exodus Privacy and ClassyShark3xodus, we may find out whether an app integrates third-party software components (=trackers), even without extensive analysis. Alternative app stores such as Aurora, Yalp or /e/'s AppsStore integrate the findings of Exodus Privacy in an user friendly way. So, there you may find out before installation what trackers come with apps.
If you want to be sure to avoid trackers entirely, it seems the only easy way is to download apps exclusively though F-Droid. This alternative app store that is curated in a way that only open source and tracker-free apps are integrated. (All apps from the F-Droid store can by found in /e/'s AppsStore too. Unfortunately, for now it is not possible to hide all those apps in AppsStore that are not curated by the F-Droid team, hopefully this feature will be available quickly).
Fortunately, there is software that allows to identify and block tracker (and ads). As a consequence, those trackers become harmless as they cannot ‘call home’ anymore. Examples for such DNS-based ad and tracking blockers are Blokada or Netguard. They pretend to be a Virtual Private Network, so all data that is flowing in and out of our phones have to pass this app, which is playing the role of our phone’s guardian. The technique of those apps is pretty reliable as far as I understand. The big downside is that they can (of course) only filter what they know. So there might be trackers around sucking our private data as they are still unknown to Exodus, Blokada et al.
So what to do? In my humble opinion: Make sure that you have a privacy-enabled smartphone operating system (/e/ !!!) and use exclusively open source apps from F-Droid and open source apps from /e/'s Apps Store. If you absolutely need other apps (with trackers), the second best option is: Install as few of them as possible and make sure that you have Blokada, Netguard et al. running…