CBD isolate is a crystalline solid or powder comprising pure CBD with 0% THC. It looks a lot like artificial sugars and has no taste or smell. This type of CBD will not show up on a drug test.
CBD is one of over 60 compounds found in cannabis that belong to a class of ingredients called cannabinoids. THC is the most well know of these but not the only one with wide ranging benefits. CBD is not the only that won't get you high, but it is one of the easiest to extract.
My maker suggests CBD for tremors/seizures, calming, and anxiety. There's also evidence that it soothes the some of the negative mental effects of THC, helps with pain, and promotes neurological health, among other things.
If you're confused about how we get all the different compounds separated, think about how scientists separate blood or how heating sugar makes it break down to it's core components.