“I know that you've all really busted your asses on this project, and I really appreciate it.”
You're a manager at your company. Your team just finished a big project. You take them out to lunch to thank them. When the drinks arrive, you make a toast to their hard work.
I know that you've all really busted your asses on this project, and I really appreciate it.
When you "appreciate" something, it means that you notice it and are thankful for it.
You say "I appreciate ___" or "We appreciate ___" when you want to thank someone for their actions. This phrase sounds more clear and explicit than just saying "Thank you for ___".
An example of this is when your child confesses that she broke something, and you say:
I appreciate your honesty.
"Busting your ass" is an idiom that means "working really hard".
I busted my ass waiting tables and working construction to pay my way through college.
I know that all of you have really busted your asses on this project, and I really appreciate it.
This is a colloquial expression that you use when talking to people you're comfortable with. The word "ass" is a mild swear word. You probably shouldn't use it in formal situations.
There's also another meaning of "busting your ass", by the way. It means to injure your butt by falling:
You remember that time when you slipped on the ice in the front driveway and busted your ass?
You can say "I know that..." to show someone that you understand their feelings:
I know that you were disappointed that we didn't get a chance to visit the winery. I'm sorry about that.
I know that you must have spent a lot of time putting all of these notes together. Thank you so, so much for doing that!