Download is data coming from one computer system to another, example streaming a movie.
Upload is data sent from a computer to another, for example sending an email or posting a photo.
Download and upload work together simultaneously, this is to maintain the communication between devices, in other words you cannot have a one-way conversation.
Download capacity by design is always greater than upload due to the demand from end users wanting more download over upload, priority is given to download. At the same time to be able to maintain a download you need to be able to communicate with the server to let them know what you have received from them, and checking that it is all correct. Thus, if you were to use the entirety of your upload then you would find that your download will also suffer. This is the same for uploading being reliant on your download not being completely used.