To utilize the knowledge that life has taught you. Learn from mistakes and do better. Every action has an impact on both yourself and others and one knows in their heart when they have done their best.
It means you came in third...
You put every ounce of energy and willpower into succeeding in something.
"There is no try...do." Yoda
As everyone has already said "do" means everything you have got to give.
It's a nice way to say "Its OK to fail"
You perform the limits of your potential of what you are capable of and not worry over what others have done or exceeded you in potential.
For me it usually means I left her unsatisfied again, and when shes bored of trying, she says "well you tried your best"
When you are trying your best, there is nothing left on the table at the end of the effort. You've done everything you knew to do, the best way you could. If you don't complete the task with that feeling of exhaustion, then you are not trying hard enough.
You do whatever you can, whatever feels right. When you feel like there is nothing more you can do, you've done your best.