user.userid explained

[TECK]

can you help me understand the mechanism of this query?

$posts=$DB_site->query("
SELECT
post.*,post.username AS postusername
FROM post
LEFT JOIN user ON user.userid=post.userid
LEFT JOIN userfield ON userfield.userid=[high]user.userid[/high]
WHERE $postids
ORDER BY dateline $postorder
");

why [high]post.*,post.username[/high] and not only * ?
why is it present the [high]user.[/high] part?
what is the difference between user.userid and post.userid? why i see all the time this kind of code structure (user.userid) in vB?