According to Salesforce, Lightning app builder is a tool to create custom pages for the Salesforce mobile app and Lightning experience through its point and click action which makes the process easier and effective for the users to find all the information at a single place. Lightning app builder also functions as a one stop solution for configuring Lightning apps.
Do check out the Salesforce lightning training course from Intellipaat which includes a 22hrs online training and 44hrs of projects and exercises to practice with.
Also, do check out our YouTube video on the Salesforce tutorial.