I made an awesome program the other day, and I wanted to upload it to my Arduino. After clicking the upload button, some mean dude named avr came along and stopped me, saying:

avrdude: stk500_getsync(): not in sync: resp=0x00

All I want to do is just upload my program, but avr won't let me. He's even unintelligible, so can someone tell me what the heck he's trying to say and how to get rid of him?

i.e.:

Whenever I try to upload a program to my Arduino, I get this error message:

avrdude: stk500_getsync(): not in sync: resp=0x00

What does this mean, and how can I fix it?