The flag of Puerto Rico was first raised by people who could not fly it at home. Its debut was in a New York hall in 1895, among exiles campaigning for independence from Spain.
Designed in exile
Francisco Gonzalo Marín proposed the design at a gathering of Lares Revolution veterans in New York City in 1895, and it was unveiled on December 22 that year at Chimney Corner Hall. The resemblance to the Cuban flag was deliberate. Puerto Rican and Cuban exiles were working against the same empire, and a matching flag said so at a glance. The 1868 rising those veterans had fought in, the Grito de Lares, flew a different flag: a white cross over two blue quarters and two red ones, with a single white star. The 1895 design did not reach the island until March 1897, when it was carried in the Intentona de Yauco.
Spanish authorities banned its public display, and after 1898 so did the Americans, once Spain ceded the island at the end of the Spanish-American War. It received no official standing until July 24, 1952, when Puerto Rico became a Commonwealth of the United States.
Red, white, and a single star
Five horizontal bands, red at top and bottom, alternate with white. A blue equilateral triangle sits at the hoist with a single white five-pointed star inside it.
The readings are conventional: red for the blood of warriors, white for peace and victory, blue for the sky and sea, and the star for the commonwealth. The geometry, though, is borrowed wholesale from Cuba, and that borrowing says more than any of the individual colors do.
The one detail that still causes arguments is the blue. Over the years it has run from a pale sky blue to a dark navy, and Puerto Ricans disagree about which shade is correct.
Flying it
The flag flies at government buildings and public institutions and comes out for Puerto Rican Constitution Day and other holidays. Protocol covers its handling on and off the island, including the rule against letting it touch the ground.
Off the island it does heavier work. In the diaspora it stands in for the place itself, and it turns up constantly in music, art, and literature, at parades and at protests.
The Cuban connection
Cuba's flag is the direct source, and the two are still read as a pair of late 19th century independence symbols. Other Caribbean flags carry echoes of the same colonial and independence-era design language.
References
[1] Puerto Rico Government (https://www.gobierno.pr)
[2] North American Vexillological Association (https://nava.org)
[3] Library of Congress (https://www.loc.gov)
[4] History Channel (https://www.history.com)