Immersion allowed users to practice the language they are learning by helping to translate real-world documents from that language into one they already know. This feature was only available for few courses (see here).
The Duolingo CEO announced in June 2015 that they have stopped developing immersion. The last course to gain an Immersion tab was the English for Romanian speakers course (which reached beta phase on 2014/02/04). Around December 2015, Duolingo started an A/B test to evaluate Immersion.
After much speculation, Immersion was withdrawn on 19th January 2017.