knitting and crochet yarn gauge
These numbers tell you the size of the yarn and how many plies it has. It's called the "count system".

The first number is the size - the larger the number, the finer the yarn. (The physical sizes can vary for different fiber types, but they always follow that rule.)

The second number is the ply, how many singles make up the yarn.

So both of your examples are six-ply yarns, but the 50/6 yarn will be finer than the 34/6.

If you spin yarn, these numbers will tell you how much fiber you need to make a certain length. Fibers have a count associated with them (also different for each fiber type - for instance, cotton is 840 but linen is 300). That number is how long a single ply of yarn would be, spun at size 1, if you used 1 lb of fiber. So 1 lb of a 1/1 cotton would be 840 yds long. However, if you wanted to make 10/2 cotton, you would multiply by the fraction: (10 / 2) * 840 or 4200 yds of yarn, from the same weight. This value (yards per pound) is called the "grist", and is directly correlated with the count system number.

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