Sending files from your device to the internet or another computer. Like mailing a package - you're sending something from your location to somewhere else.
When you upload photos to social media, you're sending them from your phone to the social media company's servers.
“Upload” is a general networking action (sending data from a client to a remote system), not a single cloud service. In practice, uploads commonly target cloud storage (e.g., object storage), web apps, or APIs across all providers.