Personalization is a very broad concept. In this case, personalization equates to leveraging engagement data to build an interest profile for a user. Many of the apps you use every day leverage machine learning to personalize your feeds. Here are 4 examples:
1.Instagram created an "Explore" section that shows you pictures you’re likely to be interested in, based on your past clicks, likes and engagement.
2.Quora uses your engagement data, like the posts that you search for and comment on, to personalize your feed experience and emails.
3.Facebook uses advanced machine learning to prioritize posts from the friends you engage with most, after starting out with just a basic edge rank algorithm.
4.Etsy analyzes your search and shopping behavior to optimize your newsfeed, email and e-commerce conversion.