Usually performed by a guy who is talking to a girl at the bar and trying to assess how deep her daddy issues go. This will help the gentleman assess the ratio between the level of daddy issues and the quality of the blowjob.
A simple procedure execute by curious aliens or doctors to examine the lower rectum and other internal organs of a person/animal. They (gently) insert a tool, tentacle or finger into your anus. This allows them to feel your interior and explore it.
Kang: My name is Kang, and this is my sister, Kodos.
Kodos: Hello.
Homer: I suppose you wanna probe me. Well, might as well get it over with.
Kang: Stop! We have reached the limits of what rectal probing can teach us.
(The Simpsons - S8E1: Treehouse of Horror VII)