Uploading is the sending of data from one device to another over a network or other connection. It is the opposite of downloading, which is the act of receiving data. A network connection can upload and download data at the same time. Many home Internet connections upload data at a slower speed than they download.
Using the Internet involves a constant mix of uploading and downloading. Most Internet use involves downloading files and web pages, but every interaction with a server involves uploading a tiny bit of data. Other situations like sending an email, sharing a photo on social media, and participating in a video call also involve uploading data.
The word "upload" may also refer to the speed at which outgoing data travels over an Internet connection. Some types of Internet connections are asymmetrical, which means they do not upload and download data at the same speed. For example, a cable internet connection may have a download speed of 900 Mbps but an upload speed of only 20 Mbps. Connections that are symmetrical, like fiber networks, can upload and download at the same speeds.