Disposable income is the money you have left over each month after your expenses and debt repayments.
How it's calculated
This is calculated by taking your total income for the month and taking away all payment commitments and expenses for the month.
Why lenders want to know about disposable income
Lenders want to see predictable, stable income and that you're responsible with your finances. This gives them peace of mind that you'll be able to pay back what they lend you.
They look at average spending behaviour over a long period (between 6 months to a year, but it varies by lender). So if you have to spend more than usual for 1 month, it doesn't matter too much.