A tincture is CBD in oil (liquid) form, and you ingest it by placing a few drops of the oil underneath your tongue and holding it there for 30-60 seconds.
Tinctures are the second most bioavailable method of CBD ingestion, second to vaping. In this sense, bioavailable means the ability for your body to uptake and respond to the CBD and other cannabinoids. Expect a tincture to take 25-45 minutes for effect.
Tinctures are also easy to ingest, as you can simply put a few drops under your tongue for up to a minute. They also allow for simple micro-dosing throughout the day, which is a fancy way for saying that you can can start off with one drop and subsequently increase your dosage depending on how you feel.
One of the greatest benefits of the tincture method is that they are usually the least adulterated CBD products, because they are made with minimal processing.