TCO refers to tail call optimization, which is a useful optimization for languages such as Scheme which encourage heavy use of recursion. TCO allows the interpreter to reuse the current stack frame when a tail-recursive[1] function calls itself instead of creating a new stack frame for each recursive call.

Because javascript lacks this and many other features of Scheme/Lisp, I wouldn't really recommend using it for SICP.

[1] Tail recursive functions are recursive functions which merely return the result of their recursive call to the caller instead of further processing those results before returning. In pseudocode:

    // non-tail
    expt(b, x):
        if x == 0: 1
        else: x * expt(b, x-1)
    
    // tail-recursive
    tail-expt(b, x, ans):
        if x == 0: ans
        else: tail-expt(b, x-1, x*ans)

For these definitions:

    expt(2, 3) == tail-expt(2, 3, 1)