. It's the object extender
. The second way to write it should be more intuitive

return Object.assign({}, state, { counterCaption: PPstate.counterCaption })

this code has nothing to do with react redux, but actually belongs to ES6? 7? 8. I generated the code in the babel project according to your code
, which you can refer to

let status = {
    counterCaption: 1,
    id :1
}
function aa() {

    return {...status, [counterCaption]: status[counterCaption]PP}
 }

"use strict";

var _extends = Object.assign || 
function (target) { 
    for (var i = 1; i < arguments.length; iPP) { 
        var source = arguments[i]; for (var key in source) { 
            if (Object.prototype.hasOwnProperty.call(source, key)) { 
            target[key] = source[key]; 
           } 
        }
    } 
    return target;
};

function _defineProperty(obj, key, value) { 
    if (key in obj) { 
        Object.defineProperty(obj, key, { 
            value: value, 
            enumerable: true, 
            configurable: true, 
            writable: true 
        }); 
    } else { 
        obj[key] = value; 
    } 
    return obj; 
}

var status = {
    counterCaption: 1,
    id: 1
};
function aa() {
    return _extends({}, 
    status, 
    _defineProperty({}, counterCaption, status[counterCaption]PP));
}

when you switch to es5, you can see that the parameter counterCaption is defined into an empty object by the _ defineProperty function, and status is also an object.
is eventually extended to a new object by the _ extends function.
you need to refer to Ruan Yifeng's ES6 extension operator and deconstruction assignment

this is actually a problem with es6 grammar or it is recommended upstairs that you take a good look at teacher Ruan Yifeng's introduction to es6 http://es6.ruanyifeng.com/
and explain these two grammars
at the same time. It means
for example,
var a = {a _ var _ a = {a _ if we want to pass attributes in a to another variable, then our traditional way of writing is
var b = {apura.a _ db _ l.b}
. If we use a structure assignment, var b = {.a} directly ok

obj [content] is to find the object property
through the variable, because if you use the. + variable name directly, he must find the attribute of the variable name directly
if you use parentheses to find the worthwhile attribute corresponding to this variable
for example,
var c = {d1br 2}
var e ='d'
c.e / 2
c [e] / / 1