The board balances on one of its long edges while you stand with your feet on the wheels and the deck edge. Toeside is the easier version — most people learn it first.
The railstand is less a single trick than a base position that a whole family of tricks grows out of — rollos, primo flips, combinations with the casper. That is exactly why patience pays: it rarely lands on the twentieth attempt, but it almost always lands eventually. The typical mistake is forcing the board onto its edge. Most beginners use far too much swing instead of guiding it gently over the tipping point — less swing, more control is nearly always the right correction here. Hold the balance with small movements from your toes and ankles; big corrections tip you out of the stand entirely. Two things that get overlooked: the width of your trucks noticeably changes how the balance feels, and even small bumps in the ground shift an already narrow tipping point.