Milligrams (mg) are how the strength of edibles, tinctures, beverages and many topicals is measured. The label tells you how many milligrams of THC, CBD and sometimes other cannabinoids are in each serving and in the whole package.
That second number matters. A package might say "100 mg" on the front, but contain ten gummies of 10 mg each, or twenty pieces of 5 mg each. Always check the mg per piece or per serving.
Some general reference points, keeping in mind everyone is different. 1-2.5 mg THC: a microdose, often barely noticeable. 2.5-5 mg THC: a common starting dose for beginners. 10 mg THC: a standard single serving in many regulated markets, which can be strong for new users. Higher amounts are usually for people with high tolerance.
Flower and concentrates are labeled by percentage instead. To convert: 1 g of flower at 20% total THC contains roughly 200 mg of THC, though you do not absorb all of it when smoking.
Start low, wait at least two hours with edibles before taking more, and check the COA to confirm the product's mg per serving is accurate.