Please help to understand dynamic link building class

Hi,
Im new to ASP.NET. Im trying to develop ecommerce web application and i found dynamic link building class and i dont understand how it works.
please someone help me to understand codes marked as bold.
Thank you



public class Link
{
    // Builds an absolute URL
    private static string BuildAbsolute(string relativeUri)
    {
        // get current uri
        Uri uri = HttpContext.Current.Request.Url;
        // build absolute path
        string app = HttpContext.Current.Request.ApplicationPath;
        if (!app.EndsWith("/")) app += "/";
        relativeUri = relativeUri.TrimStart('/');
   ...