Mild, medium, and full are the words makers, shops, and reviewers use to describe how strong or heavy a cigar is. Many add steps in between, such as mild-to-medium and medium-to-full, and some use a numbered scale or a row of dots on a shelf card.
There is no laboratory test, industry standard, or government grade behind these labels, and different makers and shops do not use the same scale. A single label also often mixes nicotine strength with body, the weight of the smoke; see strength vs body. Some descriptions give strength and body separately, which tells you more.
A dark maduro wrapper or a large ring gauge does not make a cigar full by itself. The filler blend, especially how much ligero it contains, sets most of the strength. Treat the label as a rough guide and read the maker's description for detail.