They are long-term lovers who have spent years, sometimes decades, single and involved with married men. Basically the next level up from adultery, but not quite equal to a concubine. She is the usually long-term girlfriend of an already married man. Sometimes the affair is public, but just as often it is not. The reasons can vary: the most common being that the husband just doesn't care enough for his wife, especially if they are in an Arranged Marriage.
Sometimes it's actually expected within a culture, particularly for people of high rank even if kept private. Sometimes the wife can even drive her husband to this, but just as often she can be a kind woman, or even a Hot Consort. Some mistresses will be happy with their position, but others will want their lover to leave his wife and marry them.
Occasionally, the latter could even lead to attempts to get rid of the wife. Needless to say, if the wife wasn't aware of her existence and finds out, the mistress is very likely to die at her hands, if the man isn't the one to die.
Crime Fiction stories often involve this trope, as it conveniently gives everyone involved a motive for murdering at least one of the Love Triangle. There are a lot of historical examples , because European aristocratic marriages were almost always purely political.
It was generally accepted that the husband and though less tolerated , if only because accidents could wreak havoc with the succession wife would take lovers to provide the emotional fulfillment they may not be getting from their marriage. It was usually considered bad form to be conspicuous about it, but not always.
