That refers to two similar laws or relationships in logic, related to opening of parentheses:
not (a and b) is the same as not a or not b
not (a or b) is the same as not a and not b
Evelyn De Morgan was born in 1855.
Evelyn De Morgan died in 1919.
Jacques de Morgan was born in 1857.
Jacques de Morgan died in 1924.
De Morgan Medal was created in 1884.
Campbell De Morgan was born in 1811.
Campbell De Morgan died in 1876.
Augustus De Morgan died on 1871-03-18.
Augustus De Morgan was born on 1806-06-27.
Morgan De Sanctis was born on 1977-03-26.
The laws that let you remove or introduce parentheses in logic expressions."not (a and b)" is the same as "not a or not b" and: "not (a or b)" is the same as "not a and not b" Similar in set theory, with union versus intersection. For more details, check the Wikipedia article "De Morgan's law".
SOP (sum of products) gives you the answer and POS does not (Unless applying De Morgan's law)