Before studying Ruby, I thought gems were the sparkly things you put in jewelry.
Well yes, but also nope.
In Ruby, "gems" are third-party libraries. That's it. But! they have a cute name because Ruby is - wait for it - a gemstone.
Gems can be incredibly helpful when developing your apps, taking care of work you'd otherwise need to do manually. Gems exist to help you work with a database, test your code, run a local server, and salt and hash passwords.
Your gems are going to come from RubyGems.org. You'll want to download the RubyGems package manager, which will then allow you to do some handy things from the command line. Most frequently, you'll be using
to install specific gems to get going. A few other commands are listed on the Wiki page.
Gems are stored in a "Gemfile" - this file specifies the name and version of each gem you'll be using, and you can separate them into groups for production, development, and testing. This is helpful when you're doing something in development (e.g. using SQLite3) that won't work in production (e.g. Heroku won't let you use SQLite). A particularly helpful gem is Bundler - it lets you install your gems easily and keeps up with the dependencies for you.
And that's your 30 second rundown on Ruby gems.