You can select different parts of the code to be 'breakpoints', checkpoints that will stop the code at that exact moment.
When you set a breakpoint, the browser pauses everything when it interprets that line of code or event.
By slowing everything down and giving yourself a chance to examine what's happening at every spot you've set, you can keep track of exactly what's happening in the page or app.
Once all of the breakpoints are set, you can step through the code and examine the state of your page or app at every breakpoint.
Just like with elements, you can live-edit through Dev Tools, but remember to save those changes however you can so your progress isn't lost.