What does this mean?

[Alfa1]

I have an idea of what this does but the description seems meaningless:
Request Title Variable
This is the REQUEST variable that will likely appear in your visitors' address bars.
/admin.php?options/list/vw_server_settings

Please add a better description.

[pegasus]

If you use friendly wiki URLs, then you will rarely see a difference, no matter what this is set to.

When URL Format is index.php?wiki/&title=Prefix:Article,
Request Title Variable changes the 'title' in that format to something else. For example, if the Title Variable is set to wigwam, then the URL Format would be like this: index.php?wiki/&wigwam=Prefix:Article

[Alfa1]

Yes, I thought it may mean this.
I think its a good idea to clarify this.

Does this function affect vw3 to vw4 url redirection?

[pegasus]

If the titlevar in VW3 was not 'title', then you should edit the showwiki.php redirect script and change this line to the appropriate value:

$_OLD_WIKI_TITLEVAR = 'title';