The horizontal axis runs from pure black on the left to pure white on the right; the vertical axis shows how many pixels sit at each value. A dark scene stacks data to the left, a bright one to the right. There is no correct shape — a histogram describes the scene, it does not judge it.
What it does reveal unambiguously is clipping. Data piled against the right wall means blown highlights with no recoverable detail; against the left wall means crushed blacks. This is why it beats the rear screen for judging exposure: a screen's brightness and ambient light both lie, while the histogram reports the actual data.
The RGB histogram is more useful than the combined luminance version, because a single channel can clip while the overall brightness still looks safe — common with saturated reds and with skin under warm light. For video, waveform monitors and false color displays serve the same purpose with more precision.