In portugueses "catraca" means turnstile, "livre" is free. So, Catraca Livre is free turnstile.
This slang was born due to a brazilian website, which informs what is possible to do without money in Sâo Paulo, Brazil´ largest city ( www.catracalivre.com.br).
It also can be translated into a open city.
You do not have to buy a ticket for this show. It is catraca livre.