Whenever you copy and paste text, it's saved in a register. Vim allows you to operate with many registers, which are identified by letters, numbers, and special symbols.
Registers can be accessed by hitting ", followed by the register identifier, followed for the operation on said register.
Registers a - z are used for holding manually assigned data. For example, to yank a word into the a register, you can run "ayw and paste it using "ap.
All of the operations you've performed so far have used the unnamed register. If you ever need to access the unnamed register explicitly, ...