Download speed is a measurement of bandwidth, and is not actually a measurement of "speed". It is more accurate to think of download speed like the plumbing in a building. For instance, the circumference of a pipe determines how much water can pass through the pipe. The larger the pipe's circumference, the more water that can pass through, and vice versa. Similarly, download speed is how many bits (or bytes) can pass through the connection over a specific period of time.
Download speed is often measured in bits per second (bps), and occasionally measured in bytes per second (Bps). Just like the metric system, bits and bytes can be represented as smaller numbers with a prefix denoting how many powers of 10 the number has been reduced by (ie. 100,000,000 bits can be stated as 100,000 kb (kilobits) as well as 100 mb (megabits)) Therefore, if your download speed was 50 mbps, it translates to a download rate of 50,000,000 bits transferred over one second.