From the Steam Store page:
The Call of Duty: Black Ops III Multiplayer Starter Pack allows you to experience the ranked Multiplayer game mode of Black Ops III. This is a Multiplayer-only version of Black Ops III that has limited content. See below for more detail.

The Multiplayer Starter Pack gives you access to Public Ranked Multiplayer matches and features including Weapon Paint Shop, Gunsmith, Arena, Freerun, Theater, and the Black Market.

For the complete Call of Duty: Black Ops III experience, you will need to upgrade to the full version of the game. Below is a list of features not available in the Multiplayer Starter Pack.