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 device to a remote system). Cloud providers don’t have a single service called “Upload,” but many services involve uploading data (for example, uploading objects to cloud storage or uploading code to a deployment service).