What is El Salvador's president offering to the United States regarding its prison system?

El Salvador's President Nayib Bukele has offered to house U.S. criminals in his country's jails. As Bukele tweeted, El Salvador is willing to "outsource part of its prison system" to the United States. This is not a misleading headline - the offer appears genuine. Importantly, Bukele specified that El Salvador is "willing to take only convicted criminals," including those who have been convicted in the U.S. justice system. This proposal represents an unprecedented arrangement where one country would handle the incarceration of another country's prison population.