Asset Retirement Obligation

Companies that run big projects - like oil rigs - have a responsibility to clean up after themselves once the project winds down. With this obligation looming, the company needs to account for the money the clean up will cost. The firm's accountants track this future cost, a process known as asset retirement obligation.
Remember that time in high school when your parents went out of town and you threw a party? (If not, do you at least remember the dozens of movies where this happened?) You knew even while the party was happening that you would have to clean it up before mom and dad got home. During the party, you kept a mental inventory: "Okay, I guess I'll have to wash those curtains...alright, I'll have to clean vomit out of that vase...I should probably just take that couch to a junkyard and say it was stolen."
This process is basically the asset retirement obligation. Only when a company does it, they put things in dollar terms ("it will cost $X to take that couch to the dump," or in corporate terms, it might be something like "it will cost $Y to reseed that forest we just turned into paper pulp").