Assign a variable to reference another variable. Following this command altering one of the variables alters both variables. In other words this command can be used to define a kind of alias to a variable. The mechanism is the same as local variable of a function is an alias of a variable passed to the function as actual argument. The difference is that this reference is not automatically released when some function returns, but rather it is alive so long as long the referencing variable is not undefined saying undef variable in a command.
To have an alias to a variable is not something of a great value though. It becomes a real player when the 'variable' is not just an ordinary 'named' variable but rather part of an array (or associative array). Using this mechanisms the programmer can build up arbitrary complex memory structures without caring such complex things as pointers for example in C. This is a simple BASIC way of building up complex memory structures.