The term "in situ" is used in many different contexts but in the online world it primarily refers to a particular user action that can be performed without going to another window. For example, in designing user interfaces (GUI), if a computer displays an image and allows you to edit the image without launching a separate image editor, this is called "in situ editing." You may hear someone say "This Web element will be in situ" meaning "show it to me in the page."
In computer science, an in situ operation is one that occurs without interrupting the normal state of a system. For example, a file backup may be restored over a running system, without needing to take the system down to perform the restore.
The term is a Latin phrase which is translated literally as 'in position.'