Updating your PATH in Terminal (OSX)
When you ask your computer to run a program, it needs to be able to find the program's executable files. The PATH is its 'official' list of places to look.
You can have Terminal add locations to your PATH each time you open it. To set this up, you need to create a 'bash profile' ('bash' being the language that Terminal runs in), which Terminal will read each time it launches. The bash profile can list locations that Terminal adds temporarily to the computer's PATH.
These steps show how to do that, in particular for adding Ruby to your PATH after installing it with Homebrew.