Tier 1 Terran

Upgrading to Tier 1 will not only increase your bases life, but it will also start spawning different units at a different spawn rate.

Tier 1 Terran
If you upgrade to Tier 1 Terran, your base will change into a barracks and you will start spawning marines. The spawning rate is roughly 1 every 8 seconds.

An upgrade, often called a racial, for your current spawning unit can be found at your current base. In this case the upgrade is Combat Shields, which grants all of your marines an extra 30 health points.

More on Tier 1 Terran
Tier 1 Terran upgrades into Tier 2 Terran at the cost of 1400 minerals.

As with all bases, Tier 1 Terran can also cast a chronoboost to increase the spawn rate/upgrade rate/attack rate of the base itself or the nearby cannon.