You want my honest opinion after over 40 years of fishing? It means almost crap.
Each classification varies by manufacturing brand and material type. IF it means anything its how large a sinker you can use without the thing breaking.
You see those guys in the stores bending those rod tips? That's how they are judging the action of a rod and whether it suits their needs.
What suits your needs best will come with experience.
There are four basic types of fishing techniques and one of those not really suited for fresh water. Fish runs with the bait setting its own hook (salt water mainly), sight, feel, and a combination of sight feel.
The key to successful fishing with a rod is setting the hook. Once you learn to adjust your drag properly and how to keep a constant pressure on the fish, reeling it in is a very fast learned skill and one can adjust the technique easily for rod stiffness. Its that setting the hook that picks up your numbers of catches.
A very stiff rod with almost zero feel and bend works well for trolling where pretty much the fish does all the work except reeling it in. This doesn't work too well with gills or most fresh water fish with the exception of in a boat trolling.
A very limber rod I think of as a sight rod. These are good for bottom fishing or not having the rod in your hand. You can see what's happening on the end of the line in exaggerated movements allowing you to judge just when to set the hook. It of course like the shock on a car cuts back on how much you feel in your hand.
A very stiff rod cuts way back on the movement, sight thing, but greatly increases the feel in your hand allowing you to judge from feel on when to set this hook. This is a usual favorite of lure fishermen.
Of course the combination or middle of the road is a compromise giving both some sight movement and feel. Its often a favorite of fishermen who both fish by feel and sight.
For most pond fishing, I like a ultralight. By its nature being thin and small a little stiffer is better. Even a stiff ultra-light will give you a decent sight picture but still enough feel that when that game fish tugs that one good one, you know to set the hook.
Game fish tend to hit the bait or play more with it a little before seriously trying to take it than many other fish especially a gill. Being able to tell when that's a serious tug not just a strike allows you to know when to set the hook. Feel is pretty important on game fish.
A stiffer ultra-light will allow you to place your lure or sinker more accurately with some practice. Knowing where the fish are is 50% of the battle. Knowing what they will eat and when is another 40%.
Tj Story Time (everyone has a fish story or qazillion of them)
Last spring, I took my family on vacation to FL. Our car was packed full so all I could take was my ultra-light packing rod a fairly stiff four piece Eagle Claw with cork handle and a five ball bearing crappie reel. While they went to Disney my friend takes me bass fishing in his bass boat.
He's immediately making fun of my rig telling me to use his. At the same time he's insisting I use his expensive stink plastic worms, a local special. Hey, its his water but I stick to my rig but use his bait. Between heckles he's telling me "I caught a monster right there." I couldn't have asked for a better guide. Every time he would say "there", I would sling my little ultra-light right there.
I slayed him catching three to his one. The heckling stopped and by the end of the day he was swearing to buy a rig like mine. I just laughed.
You see I knew it wasn't the rig. It was the fact I was use to the rig and could put it where I wanted, knew when to set the hook, and how to reel the fish. He did all the hard part by telling where they were, what they were eating, and when they were hungry.