From what I understand, there was a brief period where British citizenship was able to be passed down through double descent. It was only permitted during that time if the British FATHER (by descent) registered the birth of his child within 1 year of the child's birth. A British MOTHER (by descent) was NOT permitted to do the same. The court ruling corrects this, but only for people born between 1949 and 1983 to a British MOTHER (by descent).
So, for example:
Mary was born in the USA to John in 1955. John, who is a British citizen by descent, was able to register Mary's birth within the year so that Mary could be a British citizen by double descent.
Lisa was born in the USA to Susan in 1955. Susan, who is a British citizen by descent, was NOT able to register Lisa's birth within the year (because Susan was a woman, not a man), so Lisa was NOT able to be a British citizen by double descent.
The court ruling now permits Lisa to claim British citizenship because Susan was unable to register Lisa as British due to what was judged as gender discrimination at the time (British mother versus British father).
And it should be noted that this ruling only affects those who were born abroad in the qualifying years (1949 to 1983) to a British-by-descent mother. I suspect that if you were born between 1949 to 1983 to a British-by-descent father who did not register your birth within the year, you are out of luck as your father was fully entitled to do so at the time and did not. (whereas the mother was actively prevented from doing so)