This and base and squared bracket parameters

This and base and squared bracket parameters

I stumbled upon this in the project I will be working on:
public int? OrgId
{
get { return base["OrgId"] as int?; }
set { base["OrgId"] = value; }
}
Clicking on OrgId, Resharper takes me to method of this signature :
public override object this[string propertyName] { get; set; }
What does base[""] do ? And what means the this[] part of the override ?
Couldn't find anything meaningful on Google.