An umbrella company is in simple terms an outsourcing of your payroll. Rather than having the cost and hassle of setting up your own limited company you can work via an Umbrella Company.
Asking these questions should give you an idea of what it will be like working with them. Your relationship with your umbrella company is important, so don't make your decision lightly.
Umbrella companies were spawned following the introduction of IR35 back in April 2000 and became hugely popular after the Management Service Company Legislation in April 2007. They are seen as one of the last remaining 'compliant' payroll methods, taxing contractors fully under PAYE, after the deduction of allowable business expenses.