Five pillars of Islam. To believe that Prophet Muhammad is his last prophet, but also to believe in Jesus, Moses and all other prophets. And Hajj is not mandatory; it's mandatory only if you are able to do it financially and physically.
Mecca is the birthplace of Muhammed. It's also known as holiest city in the religion of Islam this is because the Kaaba is in mecca. This is in the picture. In English Mecca is used to say any place that draws a big crowd. http://www.religionfacts.com/islam/places/mecca.htm