VST is an abbreviation of Virtual Studio Technology. Steinberg (of Cubase) is the developer of this.
The audio software in Cubase is enriched with plugins (programs/modules processed in Cubase).
This gives you real-time beautiful extensions that you can add to your Cubase sequences or play with it yourself.

Instead of buying expensive Studio Modules or Synthesizers, Pianos or Hammond Organs, you can purchase Virtual Instruments.
You place it on the computer as software and you often see very nice instruments with lots of buttons and controls on the screen.
The possibilities are enormous.

And..... A lot of this software is often free too.