Loisaida Avenue is an alternative name for Avenue C between 14th Street and Houston. The name “Loisaida” is supposedly Spanglish for Lower East Side, but I haven’t been able to confirm that to my satisfaction.
The area is heavily Hispanic—particularly Puerto Rican, according to what I have read—and I have in fact seen much evidence of this heritage, in the form of restaurants, gardens, music, shops, murals, people, and more.
To get there, take the L train to First Avenue or the F train to Second Avenue and walk east.