Network speed is generally measured in Mega-bits per second. One bit is the single smallest piece of data that a computer can interpret, which is either a 0 or 1. A megabit is approximately 1,000,000 bits. So, when you're referring to network speed, you're referring to the amount of data that is capable of being transferred in one second.
5 Mbps is sufficient for basic web browsing and light downloading. 7 Mbps would be decent for applications like Youtube and low quality Netflix.