i need help with variables and global coding

I've looked at quite a few demos and they are using codes like:
if(jump==1) {yvelocity=-12 jump=0;}
I don't understand how to program "jump" and codes like "walkleft"..
I would really appreciate it if some one could tell me how this works!
=]
if(jump==1) {yvelocity=-12 jump=0;}
I don't understand how to program "jump" and codes like "walkleft"..
I would really appreciate it if some one could tell me how this works!
=]