"A recommender system or a recommendation system (sometimes replacing "system" with a synonym such as a platform or an engine) is a subclass of information filtering system that seeks to predict the "rating" or "preference" a user would give to an item." — Wikipedia
In simple terms, a recommender system is where the system is capable of producing a list of recommendation with respect to an item. One of the ways to create a recommender system is through Collaborative Filtering, where the information is filtered by looking at the activity of other users. Most companies these days use recommender systems to provide better recommendations to the users.