Website monitoring is an automated process of checking availability of a site. The main goal of it is evaluation of possibility to access the site by clients. It is clear, that a site is efficient when an interested person can load the page and make a purchase or look for some information. If this action fails for some reason - the site does not execute its mission, and a client will find what he needs somewhere else.
There are many solutions of this problems, and all of them could be divided into passive and active ways. The result of monitoring is the value of uptime, measured with some accuracy. Having it, one can conclude how long is the site broken during some period of time (usually, for a year). Low uptime usually means that the server where the site is hosted, or internet connection to it, is unreliable and is required to be changed.