What does "Allow: /" mean in a robots.txt file?

Question

Please help me to understand the following code in robots.txt:

Allow: /

Answer 1

Allow is not officially part of the robots.txt specifications but the major search engines do honor it. But it is redundant as anything not explicitly blocked is allowed to be crawled.

Answer 2

It is possible to use the Allow directive after a Disallow directive to allow crawlers to visit URLs that would otherwise be blocked by the Disallow directive. See: Is it possible to block an entire site to Google then list exceptions?

As John says, the Allow directive is not standard, but it is supported by the major search engine spiders like Googlebot.