Early Transparent is one of a sub-group of Reine Claude plums (gages) which are "diaphanous" or transparent (Reine Claude Diaphane). The skin is thin and if the fruit is held up to the light it is sometimes possible to see the flesh and stone inside.
Early Transparent makes a good choice if you want to grow a gage which is not as fussy as the Old Green Gage. Cropping is heavy by gage standards (although still light compared with many plums) and it is reasonably reliable. It has the rich sweet gage-like flavour and aroma of a genuine gage, but it is also an excellent culinary variety producing a well-flavoured straw-coloured jam.