TIdSocketHandle = class(TIdCollectionItem);

TIdSocketHandle is a TCollectionItem descendant that represents a socket handle or binding added to the TIdSocketHandles collection. 

TIdSocketHandle provides properties and method needed to identify the socket handle allocated by the IP protocol stack, and to use perform functions like opening, closing, binding, sending and receiving using the socket descriptor. 

TIdSocketHandle is used in Indy to encapsulate the low-level connection and methods used to access the protocol stack in a Binding for TCP-based clients, IOHandlers, and client connections to a server.