Well basically… it means looking for sentences with new words you want to learn while you immerse in Japanese content.
Every time you learn a new word you should also have a sentence that uses the word.
It’s best if the new word is the only word you don’t know in the sentence.
That is called an “i+1 sentence”.
So, you do the thing you enjoy doing in Japanese like watching anime, or reading a detective book or playing a video game, and then when you notice a sentence with a word that you don’t know that you want to learn, you take the sentence and turn it into a flash card in Anki.
You can even use programs like subs2srs and anki addons like Morphman to automatically convert anime/movies/dramas into anki flash cards that have English, Japanese, pictures and audio and to find the sentences in that content that are i+1 for you and even order them by usage frequency etc.
Anyway the key point is that its better to learn words you find through immersion because each specific context has its own ‘core vocabulary’, so if you want to be able to play video games in Japanese for example you need to learn a lot of words that will not appear in that N2 book so you should just make flash cards for them in Anki as you come across them in a video game. There are many common Japanese words and phrases you won’t find in JLPT vocab books or the core series. Also, its better to learn the words from native content because you’ll remember them easier and understand the context the words are used in better.