The BMX handlebar stem is the component that connects the handlebars to the fork steerer tube on a BMX bike. The stem size is usually measured by the diameter of the steerer tube it fits onto, and in this case, it's 22.2mm.
It's important to make sure that the stem you choose matches the size of your bike's steerer tube and handlebars. A stem that is too large or small will not fit correctly and could compromise the safety and stability of your BMX bike.
When selecting a BMX stem, consider the length and angle of the stem, as these factors can affect the handling and feel of your bike. Longer stems generally provide more stability and control, while shorter stems offer more maneuverability and responsiveness. Additionally, the angle of the stem can affect the height and reach of the handlebars, so choose an angle that is comfortable for your riding style.