Increment global state variable, add it to e, which is the number of milliseconds the game has been running modulo 300. Then set the margin size to that value. It's a simple pseudo-pseudorandom number generator.

In case any one was wondering the difference between "s++" and "++s" (the former of which is probably more common), prepending the "++" increments the variable before returning it, whereas appending it would increment after returning the value.

The code you posted is essentially equivalent to "s = s + 1; a.style.margin = s + e % 300".

increments `s` before evaluating the expression and adds it to (e % 300)

++ will increment s before the new margin is set.