No, he's NOT the president of Venezuela.
Yes, he was the man who popularized the slogan "Yes, we can!" Only he said "Si' se puede!"
Cesar Chavez, American young people should know, was an American who 40 years ago was inspiring young people to work long, hard hours for social justice. And not only did they do so in great numbers, but they actually achieved social justice, they won victories that kept them going. And many of them are going still, having made long, enjoyable, and effective careers of it.
Chavez organized the United Farm Workers, vastly improving the working conditions of farm workers in California and around the country. The UFW pioneered numerous tactics that have been used with great success ever since, including most famously the boycott. Half the country stopped eating grapes until the people who picked the grapes were allowed to form a union.