The flag of the Polish Home Army - also known as the Polish Underground State. They were noted to be the largest of WWII resistance movements.
The symbol is called the Kotwica or "Anchor"
High level understanding from Wikipedia:
The Polish Underground State (Polish: Polskie Państwo Podziemne, also known as the Polish Secret State)[a] is a collective term for the underground resistance organizations in Poland during World War II, both military and civilian, that were loyal to the Government of the Republic of Poland in exile in London. The first elements of the Underground State were established in the final days of the German invasion of Poland that began in September 1939. The Underground State was perceived by supporters as a legal continuation of the pre-war Republic of Poland (and its institutions) that waged an armed struggle against the country's occupying powers: Nazi Germany and the Soviet Union. The Underground State encompassed not only military resistance, one of the largest in the world, but also civilian structures, such as education, culture and social services.
