Difference between JSON.stringify and JSON.parse

I have been confused over when to use these two parsing methods.

After I echo my json_encoded data and retrieve it back via ajax, I often run into confusion about when I should use JSON.stringify and JSON.parse.

I get [object,object] in my console.log when parsed and a JavaScript object when stringified.

$.ajax({
url: "demo_test.txt",
success: function(data) {
         console.log(JSON.stringify(data))
                     /* OR */
         console.log(JSON.parse(data))
        //this is what I am unsure about?
    }
});