These are an excellent way to save money on a new bike. Essentially your employer buys the bike for you, and you pay the money back in instalments though your salary. You save money because these payments exclude tax and national insurance, and so, depending on your tax status, you could save 25-42% of the price of the bike.
You need to find out which scheme your employer operates for the specific details of how that scheme works. Some large cycle retailers operate their own schemes, whereas other schemes apply across different bike shops.