A plaza is an urban park or square, or another public space where people can walk, sit, and congregate. You might, for example, tell your friend to meet you in the plaza downtown with the big horse statue.
There are plazas that are mainly open spaces for pedestrians, sometimes including statues, fountains, and benches, like Washington DC's Freedom Plaza or Union Square in New York. Other plazas are more commercial, offering room to stroll and sit but also stores and restaurants. In Spanish, plaza simply means "place," and the earliest plazas were built in Spanish colonies in South America and the East Indies.