They are looking at several things: Are bills paid on time?
Do you have court ordered judgements for non-payment?
What is debt to income ratio?(do your monthly payments take up too large a percentage of your income) Many landlords will not accept you as a tenant if the rent is more than 30% of your income, so if your monthly payments are too high, you would not be able to pay the rent.
Some companies also run a criminal history, or run the names through the national sex offender database.
You would be wise to request a full copy of the report so you can use the opportunity to check for inaccuracies and make sure you aren't the victim of identity theft. In general, you should review this report 1-2 times a year.