A website overlay is one in which the visitor to the site must make a decision and click on something or it may show that an error has occurred. They may not be able to continue onto the site until they click. A lot of websites require an email address to be entered in order to have access to the website.
This is a great tool to use in order to get your visitors to stay longer on your site and this will increase your data. All of these things affect your traffic that comes to your site. The site overlays force visitors to make more clicks on your website without really even thinking about it. They only do that because they want access to the website.