It holds the bearings which allow the front fork steer tube to pivot freely.
In bicycles, these bearings are collectively called the headset. These are usually ball bearings, although some headsets use needle bearings.
The head tube of a bicycle is sometimes designated by the fork steerer column it accepts.
This can lead to confusion, since the inside diameters are dependent on the headset standard.
For example, frames that take one inch (25.4 mm) steerer columns can have three different inside diameters for threaded and threadless headsets (not including integrated-type headsets).
The wide variety of integrated and non-standard, proprietary headsets that some frame manufacturers have created (and abandoned in some cases) makes listing all current and past dimensions problematic.
Adequate press fits are typically between 0.1 and 0.25 mm of interference.