What is passive learning? It's the idea that you can learn things (for example a language) by simply having it happen in the background. You may have heard of some people who play these language learning tapes/MP3's etc while asleep. Up till now there was some skepticism about how effective this is. In fact Passive learning on it's own has been found to be almost useless! however....
What I've found (as well as others) is that passive learning is effective at cementing a language in place. For example if I were to just be listening to music from a new language without having learned a single word then I would have probably 0% comprehension. I would understand none of it and it wouldn't be very useful. But if I've learned even 1 word that is used in that song then every time I hear that word it will be reinforced and solidify that memory. This extends to other things such as Movies or just having TV playing in the background.
I've found that passive learning becomes more effective the more you learn of a language. So at the start your time is best spent with the textbook and memorisation tools (Such as ANKI). Over time as your vocabulary increases then start to incorporate more and more passive immersion into your timetable. Great examples are Songs, Tv shows, Movies, Radio stations, talk shows etc. Anything that has the language being used.
With this the important thing is that it is PASSIVE. This means you aren't trying to decode and understand everything being said. If you were to do that all the time then that would be exhausting. Instead you allow your subconscious to pick things out. Understand a word every now and then. Your comprehension is less than dedicated study but the point is it's a great way to keep the language familiar without exhausting yourself. So... Try it out and see if it helps!