Is Free, really free?

For someone living in South America in an undeveloped country, how is it possible to earn online without any investment? I have found that the ads say "Free, no investment, no obligation" then when one has joined the site, did some tasks, then it dawns on you that it is not really free, some investment has to be made and that is even before you earn from most sites. What i want to know is: Are there any proven reputable sites where persons can earn a decent income without upfront costs and it is available to everyone worldwide?