A thickening agent is an ingredient, which is to thicken sauces, soups, gravies and stews to obtain the correct required consistency. There are many types of thickening agents such as

Roux,

Cora flour, Arrowroot or Fecule,

Cream,

Beurre manie,

Egg yolks,

Blood.

Roux

A roux is a combination of equal quantities of fat and flour cooked to a sandy texture in three different degrees, such as
White roux - without color
Blond roux- colored until slightly brown
Brown roux - colored until brown in color
Could be added to a warm liquid.

Corn flour, Arrowroot or Fecule -

These are diluted in cold/ Luke warm water, and added into the warm . and stirred until well mixed.

Cream

Cream is added into a warm liquid and stirred to the boil.

Beurre manie

This equal quantity of fat and flour kneaded to a smooth paste. Could be added to a warm liquid and stirred to the boil.

Egg yolks

Egg yolks are whisked into to cold or warm liquid, and brought to the boil.