This is a measurement of the speed that data is able to flow to your device from the source. The faster, the better.
Speeds over 5Mbps give a good surfing experience, and over 20Mbps will feel awesome. Speeds under 1Mbps can make the modern internet feel slow, and speeds under 500Kbps are painful.
Download speeds have a particularly large impact on streaming video. If the speeds aren’t able to keep up, you will experience stuttering, pauses and long buffering delays. But it will also impact how fast webpages and graphics load.