What is the meaning of average latency in Hard Disk?

I want to purchase a hard disk and i am looking to buy one. But seriously i don't have any technical knowledge regarding hard disk what are the technical specifications which make it superior as compare to other. While reading the details i encountered a point average latency i didn't understand what does it mean could anyone out here can tell me what it actually is.

Average latency also called rotational delay, the amount of time it takes for the desired sector of a disk (i.e., the sector from which data is to be read or written) to rotate under the read-write heads of the disk drive. The average rotational latency for a disk is half the amount of time it takes for the disk to make one revolution.