The Tech in the “Combat” tab allows you to unlock new Units and improve their Statistics.
To be able to hire a particular type of Unit from the “Units” tab of your Action Panel, you will need to research the relevant Tech first. This Tech is always named after the Unit it unlocks: i.e. to unlock Commandos, you must research the “Commandos” Tech. There are a further 4 Techs that relate to each Unit: Speed, Offense, Defense, and Hiring Speed. Each Tech can be upgraded to Level 20, drastically improving the relevant Unit’s statistics.
Additionally, there are Techs that improve the statistics of Combat Unit groups - Light Infantry, Mechanized Infantry, Armor, or Aviation - as a whole.