Unless the device has a way to know which headphone/IEM you're using, it's a nonsensical reference level based on voltage. You can pretty much ignore it as it's unrelated to how loud your music really is. In Europe we have some pretty stupid laws about loudness protection(good intent, stupid law) where they managed to completely ignore how headphones really work. For example, let's say the warning level is 1volt(I have no idea). You can easily find a headphone that will give you a certain loudness when fed 1V, and some sensitive IEM that will give you music maybe 25dB louder when also fed 1V. The range of possibilities is wide enough that the same warning level could be totally safe with one headphones and definitely dangerous with a certain IEM. You probably can ignore this warning.
Obviously I do not recommend to listen to very loud music. But your ears usually do a pretty good job of telling when something is loud. The only flaw is when we're in a noisy environment for a long time, we tend to get used to that ambient level no matter how absurdly loud, and we will set our music to surpasses that noise. That's dangerous.
But just look at the level setting you usually use on your DAP. Listen at that level after you've been in a quiet place for a while, and you'll feel if it's very loud or not, and if you should avoid reaching that volume setting in your daily use from now on. In the absence of proper tools to measure the output of your headphone/IEM, I believe this is the most reliable approach.
Obviously you should also try to consciously lower the volume level. like when passing in a noisy place or walking near cars, we'll often need to increase the volume setting a little. but we might not think to go back down after those loud noises are gone. Some personal effort is needed for that and could in the long run, give your hearing a few more years. And if the environment is very loud, you should give up on music in that area and maybe just keep your IEM/headphone as earplug.
