Above reply got valign(or vertical subpixel) right, you don't generally need to worry about it except for bunny hops or certain aids jumps. Vertical subpixel position isn't usually relevant to vstring except in certain needle aids jumps.
Vstrings are strings of vertical positions. Best demonstration of this is vstring tricks, like here.
Just fall off and hold left on any of these jumps, and you won't hit spikes. Fangames run at 50fps, and it's tied to the physics. The kid never touches the spike in any of those frames, even though it looks like you should move right into the spike.
There's all sorts of uses for vstring magic. The simplest are those 2, 4, 7 block tricks in that image. There's a lot more situations where you can use vstring tricks though.