generate scaffold tells Rails to create everything necessary to get up and running with topics.
topic tells Rails the name of the new model.
title:string says that topics have a title, which is a "string".
description:text says that topics have a description which is a "text". (What's the difference between "string" and "text"? Basically "text" is for strings that might be very long.)
Type this in the terminal:
This tells Rails to update the database to include a table for our new model.
Here, rails db:migrate is a command provided by the Rails framework. It uses the migration file we just created (db/migrate/201xxxxxxxxxxx_create_topics.rb) to change the database. Database migration files can be crucial to code collaboration.
You can run rails --help to see a list of all the rails commands your app currently responds to, along with a short description of each one.