I have the following JavaScript source code which I retrieved from the https://www.studytonight.com/post/capture-photo-using-webcam-in-javascript link:

function startup() {
    video = document.getElementById('video');
    canvas = document.getElementById('canvas');
    photo = document.getElementById('photo');
    startbutton = document.getElementById('startbutton');

    // access video stream from webcam
    navigator.mediaDevices.getUserMedia({
            video: true,
            audio: false
        })
        // on success, stream it in video tag
        .then(function(stream) {
            video.srcObject = stream;
            video.play();
        })
        .catch(function(err) {
            console.log("An error occurred: " + err);
        });

My question is what is happening with the following part of this source code:

.then(function(stream) {
                video.srcObject = stream;
                video.play();
            })
            .catch(function(err) {
                console.log("An error occurred: " + err);
            });

Can someone please explain what the .then(function(stream) section is doing in this particular case? Thank you.