It means when you buy 100 grams of 35% THC weed, you are buying 35 grams of THC and 65 grams of other stuff. Having higher THC doesn't always mean it will make you higher than a different strain with a lower THC.
Yes, bud material that's sent to a lab that tests out at 35% THC means the bud material is literally 35% composed of the THC molecule, by weight. That's a lot! I mean just imagine, you've got a fat, dried bud in your hand, and over 1/3 of the weight of that bud is the weight of the resin... specifically the weight of the THC in the resin. The resin, from the resin glands (trichomes), contains THC and sometimes other cannabinoid molecules in significant amounts (e.g. CBD, CBG, CBN, etc.), terpenes, and flavonoids. In a bud with 35% THC, the resin is composed mainly of the THC molecule (specifically, THCA).
Cannabis testing labs use sophisticated equipment to run cannabinoid tests on samples of dried bud material, resin, oil, or tincture. The cannabinoid test will show how much THC, THCA, CBD, CBDA, CBG, CBGA, CBC, CBCA, etc., is in the sample. Then the lab produces a report with this information which is called a "COA", which stands for Certificate of Analysis. If you buy flower or concentrates from a dispensary, you can ask to see the COA for the product you are buying. If they give you a sheet of paper (or a PDF file) and it doesn't have the name of the lab on it, it's not authentic – the COA must be the document produced by the lab.
COAs are a big thing in the cannabis industry. If you are looking for a specific type of high-CBD hemp seed, for example, you would ask to see the seller's COA for that seed. This gives you some confidence that the seed will produce plants with flowers that contain the amount of CBD that the seller is advertising for that strain of cannabis. A lot of shenanigans goes on with COAs, so caveat emptor! (
Phenotypes complicate this... read on...)
If you are a home grower looking to buy seed from a big-name seed seller, it's not very common for them to supply COAs. You can ask to see one, but probably more often than not, they won't be able to provide one. Instead, you'll need to fish around on the internet and look for strain information websites... for example, google for: Girl Scout Cookies strain info. Some sites will come up and you'll be able to check the known THC (and/or CBD) levels for that strain. Sometimes seed for a given strain will produces more than one
phenotype, meaning genetically different characteristics, inclding sometimes differences in the amount of THC (and/or CBD, etc.) The number of phenotypes is typically one, two, or three. Different strain information website often have conflicting information, so it's a tricky situation.
Once you get a good sense of the expected percentages of cannabinoids for a given strain, then you can shop around at the seed sellers and see what THC% they are advertising, indica/sativa percentages, etc. Does it match up with your research? If so, you have some confidence about what to expect when growing out the seed and harvesting the buds. If you are a medical cannabis grower, your state may allow you to have access to the testing labs, so you can have your buds tested.
Over time, it gets easier and easier to recognize quality seed and dependable sellers.