Don't feel bad - I had to look it up. Article from six months ago:

Title 42 is a clause of the 1944 Public Health Services Law that "allows the government to prevent the introduction of individuals during certain public health emergencies," said Olga Byrne, the immigration director at the International Rescue Committee.

Rarely used over the past few decades, the Trump administration used an interpretation of Title 42 to issue a public health order during the COVID-19 pandemic to rapidly expel migrants at the border, citing concerns over the spread of the virus, without giving them a chance to apply for asylum, Byrne said.

"U.S. law says that any person in the United States or at the border with the United States has a right to seek asylum," said Byrne.

https://abcnews.go.com/US/title-42-amid-backlash-biden-administration-defends-trump/story?id=80149086