To be able to debug an application is very important to understand the flow of an application's logic and data. It helps solving logical bugs and adds value to the programming experience and code quality. Two popular gems for debugging are debugger (for ruby 1.9.2 and 1.9.3) and byebug (for ruby >= 2.x).

For debugging .rb files, follow these steps:

1. Add debugger or byebug to the development group of Gemfile
2. Run bundle install
3. Add debugger or byebug as the breakpoint
4. Run the code or make request
5. See the rails server log stopped at the specified breakpoint
6. At this point you can use your server terminal just like rails console and check the values of variable and params
7. For moving to next instruction, type next and press enter
8. For stepping out type c and press enter

If you want to debug .html.erb files, break point will be added as <% debugger %>