A Dowager is the widow of a Peer, more specifically, she held a title by by virtue of marriage (rather than inheriting it herself) and gets to keep the title after the death of her husband. However since the title itself is passed on, she becomes a Dowager.

For example, the Dowager Countess of Grantham used to be married to the Earl of Grantham, but now her son holds that title, since the death of her husband.

Kindred do not become Dowager unless they either became kindred while already being a Dowager as kine, or if their husband was a Kindred, who held a Kindred title, and then met the final death.

A Dowager ranks at the same level as she did before the death of her husband, but her Dowager title ranks below current titles of equal rank, so the Dowager Countess of Grantham ranks behind the current Countess of Grantham.