The cincture is a crucial part of the clerical wardrobe, but one that isn’t often noticed by the public.
It is essentially a rope tied around the waist whenever an alb is worn. It has practical utility — it keeps the flowing alb in check — as well as theological symbolism. It represents chastity, continence, and self-mastery.
Today, the cincture is a fairly simple piece of rope (maybe made of cotton, wool, hemp, etc. The ones made by Granda are 100% rayon), but it was not always so. Early cinctures seem to have been more richly embroidered and decorated.
The standard cincture is white, but clerics are permitted to change it up depending on the liturgical color of the day.
The vesting prayer for the cincture is as follows:
Gird me, O Lord, with the cincture of purity, and quench in my heart the fire of concupiscence, that the virtue of continence and chastity may abide in me.
Some institutes and organizations have even adopted a sort of cincture as a symbol of chastity; the Angelic Warfare Confraternity is probably the best-known of these.
You may have noticed your local bishop wearing a red band of fabric around his waist. Bishops wear red, priests can wear black, and the Pope wears white. This sash is called a fascia, and it derives from the cincture. Its purpose, symbolism, and utility is the same, reminding wearers and viewers of their sacerdotal vow of chastity.