How to interpret axis of Histogram and distribution curve?
First figure shows Frequency(Y-axis) distribution over varied values of Line data(X-axis). Similar information gets conveyed by your second figure as well, but second one provides a deeper insight to frequency fluctuation over smaller bins of Line data. Additionally in second figure, various types (Lognorm, Exponential, etc.) of distribution gets line traced on the population distribution. This additional line helps to figure out co-ordinates on this grid where there is a spike or dip in Frequency values based on Line data.
Every distribution has a spread or else mathematical computations won't be feasible for any kind of inference/analysis. For Line data, in both your plots, X-axis takes that responsibility. Unit (m specified by you) associated to values shall entirely depend on nature of your Line data feature.


Also, I remember writing a series of articles and creating a GitHub repository few years back on Statistical data visualization with Python and Seaborn which might be of some help. So please feel free to check, if required.