Flying by the seat of your pants is old aviation slang for flying by instinct, by how it feels rather than using your (at the time rudimentary) tech instruments. In the writing world, starting your book with no plot pre-planning, just putting pen to paper and seeing where you go, is called pantsing. The opposite of this is called plotting, where you plan out your entire story. This can be as simple as a five page summary or as intricate as a 50 page, seventh-level alphanumeric outline.
Plotting vs. Pantsing is a favorite hobbyhorse amongst craft books. Some people swear by one method, some by the other. Some are adamant that one method is better than the other, which can even escalate to the other being labelled "wrong."
I prefer to view it as a spectrum, with total plotting at one end and total pantsing at the other.
I also believe that the method you use to create is innate. If you're a born pantser, trying to make yourself into a plotter will only kill your creativity. So find where you are on the spectrum and go with it.
Ignore anyone who says you ought to write in a different manner. Do whatever puts words on paper for you.
Everyone says they want to write a book. Some do begin. Comparatively few actually finish.
Getting the story down is the biggest hurdle you'll face. This is because putting words on paper is a lot harder than most people realize. So do what gets your idea out of your head and into tangible form.
Now I will say, if you're writing something with multiple sub-plots in addition to your main story, you probably should find a way to keep track of them so all loose ends are tied up at the end of the book. Whether that's an outline or a checklist or whatever is up to you. But if you mistakenly leave something hanging, you can bet an angry reader will mention it on Goodreads. And Amazon. And everywhere else.
So the answer to the question - do you need to know your entire plot before you start writing? - is no. Not if you're a pantser.