A Probe is a highly skilled Telepath who is trained to read minds. For example, Quinlin Sonden (formerly Alden's Cognate) is a probe who attempted to probe Sophie's mind when she first arrived in the Elvin world in Atlantis in the First Book to meet him. His attempt failed due to Sophie's impenetrable mind.
When Sophie hears this word, her mind usually goes to human sci-fi movies.
Only a few things can hinder a probe. Ancient minds are almost impenetrable, but skilled Telepaths can manage to probe an ancient mind (like Sophie at her first encounter with Bronte). Tam shrouds his memory of his warning to Sophie in Legacy with shadowflux to hide it from Gethen. A lot of Elves have creative ways to dodge probes. Examples: Mr. Forkle, Lord Cassius, etc.