Straightforward definition of currying:

currying is the act of transforming a function that takes multiple arguments at once, in a function that takes each argument one at a time

Which means, going from

function f(a, b) {  
    return a + b;
}

to:

function f(a) {  
   return function (b) {
      return a + b;
   }
}

At the calling site, this means going from:

f(a, b)  

to

f(a)(b)  

Source