This is from the scene where Luke attempts to use the force to lift his X-Wing out of the swamp, but he fails and gives up.

This quote is one of the most notable from the original trilogy (in my opinion). It's simple but I think it speaks volumes.

Yoda believes that there is no such thing as "try" because the word itself has a negative connotation. Simply 'trying' to do something teases at the idea of eventually giving up. You put in the work and you either end up failing or succeeding in doing something. People say that believing that there is no try means being arrogant or cocky, but that is not the case here. Basically, Yoda wants Luke to be determined and trust in not only the force, but his abilities as a  Jedi. Luke has to surrender all of his thoughts of doubt and fully commit himself to the force. It is only then that he can truly "do" the task at hand.