In this video ,you will be enabling Lightning experience in your developer ORG. This will make sure that your can switch to lightning experience and start developing lightning custom components. Go through the whole video and pause wherever necessary.

For security purposes, Lightning components now require you to define a custom Salesforce domain name for your organization. If you’re using Lightning components today, you might need to take action to avoid having Lightning components-based customizations disabled when Winter ’16 is released.