A flash is for horses who have a tendency to go with their mouths open thus evading the bit, it is used to keep their mouths closed, however if you have not used this before then please be careful not to do it up too tight as if wrongly fitted it can really hurt and damage the bone at the bottom of the nose.  You know you will have the correct bit if the horse seems comfortable with it and is happy in their work with no head throwing etc and if you get the desired result from your bit.  I recommend you seek advice from an experienced person e.g. an instructor or an experienced friend whom you trust.  You may also find a book about bits and bitting useful, I think there is a book titled "All about bits and bitting" which is either one of the threshold books or one of the Allen photographic guides and costs around £4-£5.  I hope you get on ok, good luck with your new friend.