lightning:progressBar component displays the progress of an operation from left to right, like a file download or upload.

Here is an example loads the progress bar on load of the component, and in the JS controller that changes the value of the progress bar.

Lightning Component:

<!--ProgressBar.cmp-->
<aura:component>
     <!--declare handler for render the JS method for Progress bar-->
    <aura:handler name="init" value="{!this}" action="{!c.doInit}"/>
     <!--declare attributes for progress bar value-->
    <aura:attribute name="progress" type="Integer" default="0"/>
    <div class="slds-m-around_xx-large">
        <!--Lightning Progress Bar-->
        <lightning:progressBar value="{!v.progress}"/>
    </div>  
</aura:component>

Lightning Component JS Controller:

({
    //Specifying progress === 100 ? clearInterval(interval) : progress + 20 increases
    //the progress value by 20% and stops the animation when the progress reaches 100%
    //The progress bar is updated every 1000 milliseconds.
    doInit:  function (component, event, helper) {
        var interval = setInterval($A.getCallback(function () {
            var progress = component.get('v.progress');
            component.set('v.progress', progress === 100 ? clearInterval(interval) : progress + 20);
        }), 1000);
    }
})

Output: