The other thing you have to worry about when buying a leader is the tippet size. So this leader is a 9 foot 5X. 5X is the tippet size and that refers to the turnel end the leader - the thin end, the end that you tie your fly on to. And in a knotless leader it's an integral part. But if you were tying your own knotted leader, it would be a separate piece separated by a knot. So this is a 9 foot 5X leader, standard trout leader. If you remember nothing else and you're going trout fishing, remember to ask for a 9 foot 5X.
The ability of the tippet is to support any given size of fly is about 3 fly sizes, so this 9 foot 5X leader I can use with a size 14, size 16, or a size 18 fly. If my tippet gets too heavy, let's say I am trying to use my size 14 fly with a 2X tippet which is quite a bit heavier than this 5X, I may not even be able to get the tippet through the eye of the fly to tie it on. But the other thing that heavy tippet is going to do is it's going to be too stiff and unnatural compared to the size of the fly I'm using, so the fly isn't going to look natural on the water.
On the other end of the scale, if I take that size 14 fly and try to use it with a 7X tippet which is very very fine, almost like hair, that tippet isn't going to straighten very well. The fly's air resistance might even twist the tippet in the air and spin it around. So you need to balance the size fly you're using with the tippet size.
You can see the recommended tippet size to fly size relationship in the chart we have in the Learning Center. I would advise that you either print this out and keep it in your wallet or memorize it. It will save you a lot of heartache on the stream later.