Everyone understands + - * /, but % usually surprises people who haven't done much coding before.
% is modulo. When you first learn to divide in primary school, and you do remainders, well, modulo is the remainder. That's it.
Ask a 10 year old kid '5 divided by 2' , they'll tell you '2, remainder 1'. In the same way, 5 % 2 = 1.
This is useful because it sets up loops; when the first and second number are the same, the remainder is 0. If you're doing modulo by 5 on an input that smoothly increases, like @ptnum or @Frame, you'll get a sawtooth waveform that goes 0 1 2 3 4 0 1 2 3 4 0 1 2 3 4...
@P.y = @ptnum % 5;
Modulo works on fractional values too, so you can give it a float input, and smoothly varying float input like @Time, you still get a sawtooth pattern:
@Cd.r = @Time % 0.2;
Lots of effects involve setting up loops, modulo gives you an easy way to setup inputs or outputs that loop.