Font weight
How thick a typeface is drawn, from thin at 100 to black at 900.
See it in Text flow. Switch Weight between Light, Regular and Bold.
In plain words
Many typefaces come in several weights, from thin to very heavy. Weights are numbered in hundreds: 400 is regular and 700 is bold. Heavier weights draw the eye, so headings often use them. Reading text is usually 400, which stays clear at small sizes. Light text on a dark background can look thinner than it is.