When Google crawl the web they use a variety of standard automatic custom "web browsers" that they call "bots" (short for "robot"). These bots may not see your web-page in exactly the same way as you do when you use a web browser like Firefox or Chrome. "Fetch as Google" is a way to see how your pages look to the Google bots. Think of it as viewing the web though Google's eyes.
Google prefer to crawl pages on their own schedule.
If you want to signal to them that your content has changed and needs to be re-crawled you can use the "Submit to Index" tool.
Please note that I am assuming your question implies that you specifically wish to use the "Submit to Index" tool (which forces you to use the "Fetch as Google" tool). Please be aware that the "Submit to Index" tool is not the only way to achieve the goal of getting your site re-crawled and re-indexed. Industry best practice on how to do this is a much broader question.
The "Submit to Index" button will not appear until you do a "Fetch as Google" and fulfill their list of criterion. This is to force webmasters to check for and fix most of the common problems that cause crawling & indexing problems for the Google-bots.
"Fetch as Google" is not instant and can take a few seconds to a few minutes.
Note that using the "Submit to Index" tool is JUST a signal to them. There are no promises about how long before they will actually do it.
What the criterion mean ...
Your fetch must have a complete, partial, or redirected fetch status.
This just means that the result they display is the "status" column when the fetch is done must be one of those words "complete" or "partial" or "redirected". If you get something else it's probably an error you need to take care of.
Your fetch must use the Desktop or Mobile Smartphone Googlebot type.
There are 4 options on the drop down list
- Mobile: Smartphone
- Mobile: XHTML/WML
- Mobile: cHTML
You must only use the first 2 with the "Submit to Index" tool.
Your fetch cannot be more than 4 hours old.
The steps are as follows ...
- YOU click the "Fetch" button
- YOU wait a few seconds (usually) for GOOGLE to do the fetch
- When done, GOOGLE displays the "Submit to Index" button
- GOOGLE waits for you to press the "Submit to Index" button
- GOOGLE adds your "Submit to Index" request to the database.
Basically Google are just saying that step 4 cannot be longer than 4 hours. I would presume that most people press the "Submit to Index" button immediately but perhaps if step 2 takes a few minutes they might get bored and wander off for a cup of coffee and forget about it for hours and then click it and discover it doesn't work or cause some minor annoyance to the Googlebot.