Most English speakers use roughly 1,000 of the 250,000 English words in the dictionary. Knowing that a tiny fraction of words will immediately allow you to have functional fluency makes learning English less daunting.
Here are a couple of other examples:
Learning to play just four chords on the guitar or piano will allow you to play hundreds of songs
Of the thousands of exercises and fitness routines, pushing, pulling, and squatting your body weight a few days per week will allow you to strengthen your body and gain muscle/lose fat faster.
What are the few most critical things you can learn/develop to leverage 80% of the results you seek?