This is usually an agreement on a sum of money to be paid, on say a monthly or yearly basis, to a service provider, regardless of the volume/type of services actually rendered. It is the opposite of piecework. E.g., as a translator, if I enter into a flat rate fee agreement with a company for, say, £200 per month, then I am committed to translating however many/little documents they give me to translate, while always receiving the same monthly/yearly payment or flat-rate fee.