If you have no long-term debt, you would put $0 into the monthly debt. That field is for people carrying large balances on their credit cards, that sort of thing.
Now, I can't speak to that particular site, but a general rule of thumb is 28/36. That means that you should spend no more than 28% of your gross income on all household expenses. Mortgage payments, insurance, utilities, everything. And no more than 36% of your gross income on 'total debt service'. That is, your household expenses and all other debt.
For example, if your gross income is $60,000 a year, that works out to $5000 per month. You should spend at most $1400 on your mortgage, insurance, utilities, etc. Have car payments? Well, you can afford another $400 on that, if you are carrying no other debt. Spend only $1000/month on housing? You could afford to spend more on other debt servicing.
Now, this is a rule of thumb. Some banks will give you a mortgage that is eating up more of your gross. But you'll want to be extremely careful taking on more load. My wife and I were substantially below 28%. We were comfortable, and could make an extra month's mortgage payment each year while still saving up for emergencies. But I wouldn't have wanted to go much over.
Thanks to Investopedia for the information I summarised above. You will also want to check out this article, Mortgages: How Much Can You Afford?