Upload Speed refers to how many megabits of data per second you can send information from your computer to another device or server on the internet. While downloading information is more common, some online activities need data to travel in the opposite direction. Sending emails, playing live tournament-style video games and video calling a friend require fast upload speeds for you to send data to someone else’s server.
Source: https://www.allconnect.com/blog/difference-between-download-upload-internet-speeds