Latency is the time taken by the data ( or packet) to travel from source to destination.
For example : If you are located in Australia and are opening a SharePoint site which is hosted in Seattle ( USA) and if it takes 1/10th of a second to open that page, then the network latency between your machine and the machine hosting the site is 1/10th of a second.
For SharePoint deployment we have some guidelines to consider with respect to bandwidth and latency. Below table summaries with different scenarios
With respect to backup/restore of database in SharePoint 2010, network drives with 1 millisecond or less latency between them and the database server will perform well.
http://blogs.msdn.com/b/sharepoint__cloud/archive/2012/09/20/guidance-on-latency-and-bandwidth-for-sharepoint-2010.aspx
