Think of a Unit as a "container." In the case of video advertising, the Unit contains a video player that publishes advertising and content videos with a specific visual format. Technically a Unit is represented by code.
In Viralize you will find two types of Units:
Ad Unit: Units that deliver video or banner advertisements
Content Unit: Units that deliver content videos combined with video ads
There are many parameters to configure a Unit that you will learn to take advantage of with experience.