What is this mean by this term

[Dak Ghatikachalam]

I am confused 2 posters have told me that I am top posting ,

What do we mean by "top-posting"

do I pickup the last thread from that email thread and reply ? is that right
?

What if someone is emailing from a thread while I am replying at the same
time, would  that
not happen ? Would I be getting complains again that I am "top-posting"

Thanks
Dak