Following are the instructions for stock Ubuntu 16.04. If you are using a different OS, please install the packages accordingly.
As Dictionary.com says, one of the meanings of stock is:
- kept regularly on hand, as for use or sale; staple; standard:
In this context, they mean a standard unmodified Ubuntu distribution from Canonical, as opposed to derivatives.
Here "stock" means "unmodified". You see that use most commonly in the expression "stock car"
It comes from the sense of a piece of store merchandise. If something is "stock", it is just as you bought it from the store.