What is an ahead stem on a BMX?

Answer 1

The old standard was that a front fork ended just above the top tube of the frame. Then a L-shaped bit called a (quill) stem was inserted into the fork and locked in place with an expander, and at the other end of the the stem the handlebars were clamped in place. Aheadset refers to that the fork no longer end immediately above the top tube, but continues upward a little bit. And the stem isn't inserted into the fork anymore, but it's a straight piece where one end of the stem clamps around the top of the fork and the other end clamps around the handlebars.