1. A literal meaning: Here I stand (on my feet), and I cannot NOT stand on my feet.
2. A figurative meaning: Here I stand (on my belief), and I cannot NOT believe what I believe.
In other words:
In a physical sense I cannot help but stand where I stand. Luther uses this as a metaphor, meaning that he, in a mental sense as well, cannot help but stand where he stands. He has to believe what he believes.
Finally, as this concept is applied to everybody, Luthers statement also comes to mean that human faith cannot be manipulated by power, only by persuation. A human being cannot be forced, by threats, to believe what he does not believe.