How to run or execute a Lightning app in Salesforce

A standalone app is directly accessible by going to the URL:

https://<myDomain>.lightning.force.com/<namespace>/<appName>.app

- where <myDomain> is the name of your custom Salesforce domain.

If your organization does not created a namespace, then use c in the place of <namespace>. 

For example,
https://ap1.lightning.force.com/c/LightningExample.app

Note:

1. You don’t need to register a namespace prefix. But you want to do so if you’re planning to offer managed packages.

2. Custom Salesforce Domain Name(My domain) is required in order to run your lightning app. So ensure domain name is registered. If you try to run app without registered domain, then you will get an error as "Lightning components require My Domain. Please contact your system administrator for more information".

To register your domain go to Setup --> Domain Management --> My Domain --> Enter your desired domain name and submit.

For more info, checkout the below links,

How to register domain?

Guidelines and Best Practices for Implementing My Domain