Understanding MVC tags for nopCommerce

Understanding MVC tags for nopCommerce

I am new to MVC , and in the application i downloaded and trying to debug
i see this mark up
@Html.Widget("body_start_html_tag_after")
@Html.Partial("_Notifications")
@Html.Action("AdminHeaderLinks", "Common")
What does this mean?, @Html.Partial where can I find where the value
"body_start_html_tag_after") is defined ?
And this one:
<div class="master-wrapper-main">
@RenderBody()
</div>
Where can i find what @RenderBody does?, this is in a .cshtml file.