Why would President Kennedy say that Cuba is an "imprisioned" island?

He said that because Cuba was ruled by a communist government led by Comandante Fidel Castro. In the early 1960s, the Cuban revolutionary government (it had expelled USA-backed dictator Fulgencio Batista from power) nationalized numerous foreign properties and assests, many of them belonging to US companies, without indemnization.

The US government started a trade and economic embargo against the Cuban state as a reaction. After the CIA-sponsored Bay Pigs failed invasion of 1961, the Castro government became even more radical, declaring the Cuban Revolution a Communist revolution and getting close to the Soviet Union, which announced it would buy all the Cuban sugar the US had stopped importing.