Why was the Renaissance so important in history?
http://www.google.com/search?sourceid=chrome&ie=UTF-8&q=importance+of+the+renaissance+europe
Read widely, including the 6th link. (I take it you mean the Renaissance in Europe, beginning in Italy in the 14th century or so.)
The Middle Ages were primarily marked by concentrating art, writing, architecture, etc., on what the Church approved of. The Renaissance is primarily marked by humans' moving beyond religion-based themes as their only themes. It was not sudden, but it didn't take 4 centuries, either!