What is the params, "value" and "err" of examples?

[jspark-gigworks]

// Simple example
promiseRetry(function (retry, number) {
    console.log('attempt number', number);
 
    return doSomething()
    .catch(retry);
})
.then(function (value) {
    // ..
}, function (err) {
    // ..
});

In this example, then(function (value) {}, ...) and the next function (err) {} has each parameter, value and err.
But, when I tested this code template, value and err were always undefined.
Of course, I added my own code to make success and error.

When are they used? How can I use them?
Could you please advise me with a simple example?