The 50/05 refers to Propylene Glycol (PG) and Vegetable Glycerine (VG) and they are the principle ingredients of all juice. The 50/50 means that there is 50% PG and 50% VG. 30/70 would be a juice with 30% PG and 70% VG.
Of course there is other stuff in there, flavor and nic, but they are a really small part of the juice. When you look at recipes you will see that flavor might make up 20% of the mix, but that is deceiving. Most flavors come mixed in PG and the actual flavor is only a tiny part of the flavor mix so when you add 10% of a flavor you are really adding almost 10% PG with a tiny bit of flavoring. Only a few flavors come mixed with VG or something else.
PG carries flavor and VG makes vapor. Actually they do both but each is better at one thing. Those looking for clouds will generally use a high VG liquid and if you want more flavor go for a higher PG.
VG is also thicker and a juice high in VG may give dry hits in a clearo. A little water or alcohol will thin it but in general high VG is used in drippers to get the big clouds.