May Holmes is the deceased mother of Sherlock & Mycroft Holmes and the ex-wife of Morland Holmes. May and Morland divorced when Sherlock was young, and, having signed a prenup, Morland was given full custody while May was given little visitation and a small allowance.
May was mentioned by Sherlock in the episode Who Is That Masked Man? in season 4. In that episode, it was revealed that May was a drug addict, like her son. She is mentioned on numerous occasions throughout the series, mainly by Sherlock.
In Season Five Sherlock is not well, he is having problems with focus and memory, and he starts to have hallucinations. The hallucinations are of a woman that looks like his mother, and she begs him to tell Joan about his troubles and seek help.