Stack height measures the vertical distance from the center of your bottom bracket to the top of the head tube—essentially, it determines how tall the front end of the bike feels. A higher stack puts you in a more upright, comfortable position, which is great for long rides or riders with less flexibility. A lower stack gives you a more aggressive, leaned-forward position for better control on steep descents and faster riding.
When choosing, think about your riding style and body. Trail bikes typically have a stack-to-reach ratio around 1.4–1.5. If you struggle with back or neck discomfort, lean toward a higher stack. If you're chasing speed and have good flexibility, a lower stack might suit you better.
Keep in mind you can fine-tune your stack with spacers under the stem or by changing stem angle, so you're not locked into exactly one position. Just don't ignore stack entirely when comparing bikes—two frames with similar reach can feel very different if their stack heights differ significantly.