Lois Whitley is Karen Walker's mother.
Lois was estranged from Karen for many years, due to their difficult relationship based on Lois' tendency to rope Karen into her moneymaking schemes as a con artist. Prior to her actual appearance, Karen has been telling people that she has had her mother committed to a mental institution against her will. They reconcile finally after Jack intervenes and surprises Karen for her birthday and they agree to do one last con on a dying rich man.
After Grace decorates Lois' apartment on a budget, she sells it for a much greater price to buy a McDonald's in Tokyo. This inspires Will and Grace to start their own apartment flipping business during the sixth season.
You know what they say-- "Beer before wine makes you feel fine. Wine before beer makes you feel queer." — Something Borrowed, Someone's Due