In general (I didn't want to put this long formula in the title): $f(a_1,a_2,\dots,a_k)=g(x_1(a_{i_{1,1}},\dots,a_{i_{1,n_1}}),x_2(a_{i_{2,1}},\dots,a_{i_{2,n_2}}),\dots,x_l(a_{i_{l,1}},\dots,a_{i_{l,n_l}}))$, where $l
Basically, $f$ can be "separated" into a function $g$ applied to the result of $l$ functions which take disjoint subsequences that form a partition of $f$'s arguments.
A concrete example: $f(a,b,c,d)=ab+cd$; here $g(x,y)=x+y$, $x(a,b)=ab$ and $y(c,d)=cd$. A function that wouldn't be separable like that would be $ab/c+cd$.