Illegal Aliyah during the British Mandate, when there was a very limited quota for immigration. "Aliyah Bet" is to distinguish from the legal aliyah, "Aliyah Alef." "Haapalah" is a connotation of will power, referring to an episode in Tanach when a group from Bnei Yisrael tried to head towards Israel even though HaShem told them that their generation will not enter.