Probes are one of the three critical parts of the Managed Availability framework (monitors and responders are the other two). As I wrote previously, monitors are the central components, and you can query monitors to find an up-to-the-minute view of your users’ experience. Probes are how monitors obtain accurate information about that experience.
There are three major categories of probes: recurrent probes, notifications, and checks.
Read more at source: http://blogs.technet.com/b/exchange/archive/2014/08/11/managed-availability-probes.aspx