This function is used to create or update a Site Setting in database label. Basically it creates a new entry in sconfig table. if key value exists the it update previous entry.
App::Config()->setSiteInfo([Key],[Value])
Note: sconfig table belongs to Config model.
// Create a new entrty
$config = App::Config()->setSiteInfo('store_max_qty',5);
// If we run again it will update previous entry
$config = App::Config()->setSiteInfo('store_max_qty',10);
See also: Setting siteInfo