As previously mentioned it refers to a list of things to do before one dies. It originates from the term 'to kick the bucket' , itself a 17th century term meaning'to die'.
I've just been to the Great Wall of China - I can scratch that one off my bucket list.
A list of things you want to do before you die. I believe the term comes from a Rob Reiner movie called "The Bucket List" in which 2 terminally ill men (played by Morgan Freeman and Jack Nicholson) go on a road trip to do the things they wanted to do before they died (kicked the bucket)...or it could be that the term pre-dated the movie (?) and that was the reason they called the movie what they did.
Sadly, the old man passed away with his bucket liststill containing several unfulfilled desires.