activitycontext.getextension

Hello - What does ActivityContext.GetExtension<T> do? MSDN says it "returns an extension of the specified type."

Return Value
Type: T
The extension of the specified type if present; otherwise, null.

http://msdn.microsoft.com/en-us/library/dd486210.aspx

However - I'm not clear on what an "extension" is within the context of an ActivityContext. It doesn't appear to represent a file extension since it's an "extension of a type." Can you provide some clarity on this concept for me?