Download speed is the rate that information is received over your Ethernet line. For example, the time it takes to download software from the internet onto your computer is determined by the download speed available.
Upload speed is the rate that information is sent over your Ethernet line. For example, the time it takes to send high-resolution images from your computer to another is determined by the upload speed available.
On an Ethernet line, upload and download speed is symmetrical, this means if your line is 20 Mbps, your download speed is 20 Mbps and your upload speed is the same at 20 Mbps.