Elevated garbage bin
They are garbage bins, elevated to keep out pests, and usually locked so that people won't pick through the garbage and spread the trash around. Obviously this one is empty at the moment.
A couple related observations:
Garbage pick up is usually 3 times a week in Brazil, and even operates on New Year's eve. I indeed saw garbage being picked up at 11 pm on New Year's eve. Back in the US and Canada, you're lucky if your garbage is picked up even once a week.
Brazilian households produce much less garbage than North Americans. I'd estimate half as much as a similar size household in Canada or the USA.