Pretty decent deal for 3 months audible membership - 1 book/credit per month for £3.19 per month (60% off - usual price is £7.99 per month for this type of membership). When/if you cancel at the end of the 3 month period, there's a good chance they'll offer you a reduced rate for a further 3 months to keep your custom. Or some people get offered a free book credit, whereas others get £10 credited to their account.
Judging by the terms and conditions, it doesn't look like it's just applicable to new customers (see pic in first comment below). I haven't had the chance to ask customer service, but I'd guess this means you can use the code if you're at the end of your current membership.
Code expires on 4th September.