Knowledge
The goal of any Deadnaut campaign is to explore the galaxy and send the information back to Earth. In every mission, your Deadnauts will collect units of Knowledge. Some knowledge will be gathered passively - such as walking into a room you've not seen before - while others might result from investigating clues.
Library
Sometimes, gaining new Knowledge will reveal some interesting information in your Library console. For example, you might learn that an enemy you just encountered has a specific vulnerability to beam weapons.
Investigation
The amount of Knowledge obtained can be increased by the Investigation skill. This can be active, such as using a Deadnaut with a high Investigation score to examine corpses, or passive, such as putting a Deadnaut in the same room as a threat.
Credits
Primarily, your Knowledge is a form currency. You can trade it back to Earth, who will offer some new technology in return.
Once you've made a trade and received the schematics, you'll be able to fabricate as many items as you want.
The other important use for Knowledge is cloning. If a Deadnaut is killed - and you have at least one Deadnaut still standing - you can trade some Knowledge and bring the Deadnaut back to life.