Base is a highly reduced meat or vegetable stock that is either a concentrated paste or dehydrated into a powder. Either way, it's very concentrated and the commercial bases almost always have a ton of MSG, salt, additives, preservatives and sometimes artificial colors and flavors. There is a base called "Better than Bouillon" that has some nasty additives like disodium guanylate that combined with the others mask MSG in their ingredient list. Nasty stuff. I'd avoid base if you can.
Usually a recipe will call for something like "2 t. chicken base and 2 c. water". Anywhere you see such a thing, IMO, you are better off making your own stock with wholesome ingredients and using that in its place. The above example would just be 2 c. chicken stock. Good luck!
Life is too short to eat processed, artificially-colored, chemically-preserved, genetically-modified food. Or maybe that IS why life's too short.