If someone has adverse data then this will generally mean that at some time in the last six years, they have failed to maintain a regulated credit agreement I.E. mortgage, loan, HP or credit card. Information relating to how you have conducted the repayment of any agreement is shared with the credit reference agencies such as Experian and Equifax and this information is then made available to any new credit provider you may approach in the future.
There are different levels of adverse data all of which will remain on your credit file for a six year period, the levels are late payments, this is where you should have made the agreed payment on the 28th of the month and did not pay it until the 10thof the following month, this could indicate to a lender that any future borrowing could be unaffordable.