It means that the British Embassy in Abu Dhabi has processed your application and dispatched it to the Visa Application Centre (VAC) where you submitted your passport. Once the VAC has received your passport, they will contact you about collecting it (or courier it to you, if you have availed this service). They have reminded you not to make any travel plans because they don’t advise the outcome of your application by email: you will have to open the sealed envelope to ascertain if your application was successful.
Credit:stackoverflow.com‘