In English for example, learning the 800 most common words, you can understand 75% of the language: https://www.bbc.com/news/world-44569277.
I'd love to start fresh on a new language, take 800 new words, try to learn 10 a day, and see where I get after 3 months. Can I really understand 75% of text if I have perfect recall of those 800 words?
Like other people said here, understanding will probably still be limited, esp. in writing. But expressing even complex things becomes easier.
E.g. instead of saying "Do you have medication against migraine" at a pharmacy you could say "Do you have something for pain here" while pointing at your head.
This is what we call fluency, and starting at 800 words I would argue you have basic fluency in the language. And also regarding understanding spoken language – those words might be enough to express that you haven't understood something and ask people to simplify.
Words are not enough, though – pronunciation and grammar also play their part.