Outgassing is the release of trapped gas or vapor that was previously dissolved, trapped, frozen, or absorbed in the solid. Outgassing can include sublimation and evaporation which are phase transitions of a substance into a gas, as well as desorption.
How does it relate to my vacuum system?
In High (HV) and Ultrahigh (UHV) vacuum systems the major contributor to the gas load is desorption of gases and vapors from the vacuum surfaces (in the absence of injected gas).
Why does it matter?
The effects of Outgassing can impact many pieces of equipment in the chamber. Released gases can condense on sensitive equipment like lenses, vacuum viewports, and other substrates, rendering them inoperative in some cases.
What are the main sources?
All materials and surfaces are active sources of outgassing. Some materials are worse than others. The worst are plastics, elastomers, and glues; porous ceramics and porous metals; most lubricant, heat transfer greases, and our own fingerprints, hair, skin cells, dust, food, etc. The most predominant gases / vapors outgassing from surfaces are water vapor, oil and grease vapor, solvents, organic materials. In the upper reaches of Ultrahigh (UHV) vacuum, the stainless steel used to manufacture much of the chamber itself outgases hydrogen and carbon monoxide to some degree.
The best way to reduce outgassing is to use materials known to offer very little outgassing in the first place. 304 and 316 Stainless steel is known to be a great material to use in the construction of a vacuum chamber. For electronics, PEEK (polyetheretherketone) low outgassing thermoplastic is great for connectors and Kapton insulated wire is a great insulator for its low outgassing characteristics. Outgassing.NASA.gov has a great list of materials with minimal outgassing.
To further reduce outgassing during vacuum pump-down, one must cause adsorbed or absorbed gas / vapor to enter the gaseous phase so that it can be pumped out of the chamber. This can be achieved by baking (heating the chamber and its components), break the bonds between surface and adsorbed vapor with short wavelength UV light. Baking (AKA bake-out or baking out is the most common within the vacuum industry).