Source code is the code before it has been compiled. Humans can read the source code, seeing what algorithms are in it, seeing what comments are in it (might be very telling), they can see how it works, etc. Computer chips (CPUs) don’t talk in this language and thus it must be converted (compiled) into machine language that CPUs understand which is all 1s and 0s.

If you just managed to get your hands on the compiled code, it may take you forever to reverse engineer how the code works. You won’t get the comments either.

It’s kind of like trying to recreate a magic trick. You can see how the trick works and start to figure out how you might be able to recreate the trick, but with so many things hidden from you it may be impossible for you to understand how the trick really works.