From VerbotWiki

I'm sorry I didn't catch that? (Alternate Text Variable)

The Alternate Text Variable allows you assign alternate text in case a variable has not yet been defined.

[name:alternate text]

Example:

Rule Name: What is my name?
Input: What is my name?
Output: As far as I know, your name is [name:Friend].

In the example above if the User asks "What is my name?", the Verbot will respond in one of two ways depending on if the Variable name is set.

If the Variable name has been set, the Verbot will respond with the set Variable. For example if name was previously set to "John", the Verbot will respond with "As far as I know, your name is John"

However, if the Variable name has not been previously set the Verbot will respond with: "As far as I know, your name is Friend."