The upload speed is the speed by which your connection transfers data from your device to the internet (e.g to websites or social media sites or apps).
Whenever you view or interact with a internet based application or website, there is a two way conversation taking place between the application/website and your device (PC, laptop, phone, etc).
To display the content from a website on your device, download speed is used. When you navigate a website and click on different links, your upload speed is used to send your clicks to the website so that the content behind the link can be displayed.