Head tube angle is the angle of the front of the frame relative to the ground, and it has a big impact on how your bike steers and handles. A "slack" angle (lower number, like 63–65°) makes the bike more stable at high speeds and on steep descents—great for aggressive riding. A "steeper" angle (higher number, like 67–69°) gives you quicker, more responsive steering, which is ideal for climbing and tighter trails.
The right angle depends on your riding style. Cross-country bikes typically run 67–69° for efficient climbing, trail bikes sit around 65–67.5° for a balanced feel, enduro bikes range from 63.5–65.5° for descending confidence, and downhill bikes go as slack as 62–64°. Even a half-degree shift is noticeable.
Don't assume slacker is always better—a bike that's too slack will feel vague and hard to steer on climbs and flat trails. Think about where you actually ride most, not just the gnarliest descent you might encounter.