A cash advance is a purchase of your business future receivables/sales/income. The business pays back the money lent through a fixed percentage of its daily sales via the credit card processor or as an ACH from your business bank account.
When paid back as an ACH it is a fixed payment amount daily (sometimes weekly) that is pre-calculated as an estimated percentage of your daily sales. When paid back through the credit card processor it is a percentage, so the payment varies based on your sales for that day.
A cash advance is easier to obtain than a traditional bank loan or term loan. Your credit score can be less than perfect and you need just the last 4-6 months of bank and/or credit card statements along with a one page application for an approval. You don't have to use collateral to secure the debt and you can have the money in your account within 24-48 hours.