It only means something relative to yourself. Over time you'll be able to correlate things like effort and how you're feeling to heart rate ranges, then use your current heart rate on a ride to help pace yourself or perhaps interpret your effort vs performance if it's changed.
As an example for _me_ I might head out at the start of a ride too fast for a long ride's pace but I feel great so I don't think I'm going too fast. However, checking my HR and seeing it's for instance 180-ish, I know I'm riding too hard.
I know what heart rates I'm ok at for longer rides, vs shorter efforts, so rather than just relying on feel, I can look at a number as a more objective measure.
It can also be something to explain when what you're used to seeing is now different. For instance, you might think it's higher than it should be, which might be due to something about your hydration or how hot it is out.