Sport fishing is not complicated. Most people still fish to have fun. However it is important to keep the balance between the rod, reel, line and the lure. That word — balanced — is a bit of a moving target, but a good starting point, and can be tested and demonstrated in your local fishing store.
If you shopping for fishing line, we assume you already have rod and reel.
Line is rated for how much weight it takes to break (test strength), and bigger is not always better. Line size should follow the rod and reel manufacturer suggestions, and should roughly match the weight of the species you are fishing for.
When your local fishing store doesn’t sell the line per meter and measured spooling is not available, refer to reel capacity vs line diameter.
Just don’t be surprised if you will spool more Mahi Mahi line than reel data is suggesting.