A handle is a value that refers to some resource maintained by a system.
In Tcl, handles are simply names that are used to identify a particular resource maintained by some subsystem. Here is one example:
set chan [open /etc/motd r]
The value returned by open is a handle that identifies a specific channel to commands like read. It is an opaque handle because there is no way to use the value to directly manipulate whatever it refers to.
Because of the Tcl data model in which everything is a string, handles are pervasive. Tcl provides a number of types of objects that are manipulated at the script level via a handle:
The names of all such Tcl objects are opaque handles: They refer to things that can not be directly accessed at script level. The purpose of an opaque handle is to provide an interface or reference to some facility while keeping the implementation details out of view. It is a black box, the contents of which a user of the handle need not be concerned with. This is particularly useful in Tcl because a resource like a channel isn't really amenable to direct representation as a value. In contrast to values that are handles, other values such as string, list, and dict, contain all the information necessary to generate the correspondig internal representation.
Languages like C provide pointers, which are handles to locations in physical memory as through which values and data structures can be directly dereferenced. When this is undesirable, an opaque handle is implemented by casting one type of structure to another type that has fewer members. The two types have the same member layout at the beginning, but the additional members of the full type are not available to the opaque type. Higher-level languages like Tcl do not provide direct handles to physical memory locations, but to more logical structures and facilities. To summarize, in C a handle is used to access a resource directly, whereas in Tcl, a handle is used to identify to a subsystem a resource which the subsystem then can access.
The closest that Tcl handles come to being references to data structures is Tcl values themselves, to which are affixed an internal representation which may indeed be some kind of data structure. For example, a list, like all other values in Tcl, is a string, and a command like lindex generates an internal representation that gets attached to the list value. In this way every Tcl object can carry around with it an internal representation that a command may manipulate. Thus, every value is an opaque handle.
In Tcl, object systems surface classes and instances as commands. The name of the command for an instance is often stored in a variable, so there are actually three layers of handles in this example:
$myobject mymethod
The first handle is myobject, which is the name of a variable, which is "passed" to th $ operator to retrieve a value. The second handle is the value, which is the name of a command. The third handle is the command itself, which is the interface to the instance.