The modulus operator takes a division statement and returns whatever is left over from that calculation, the "remaining" data, so to speak, such as 13 / 5 = 2. Which means, there is 3 left over, or remaining from that calculation.
Modulus is a term used for absolute value in complex analysis, and also a term used for the thing-being-divided-by in remainder arithmetic (actually called modular arithmetic).
"The Modulus is the remainder of the euclidean division": According to the Wikipedia article you've referenced, the modulus is the divisor in the modulo operation, not the remainder: "the modulo operation returns the remainder or signed remainder of a division, after one number is divided by another, the latter being called the modulus of the operation."
In mathematics "modulus" % is not a well-defined arithmetic operation. At best it might be considered a function that returns an equivalence class rather than a number. Mathematics treats "mod" as an equivalence relation. Even in a computer programming context you can easily verify the operation is not commutative, e.g. 2 % 4 is not 4 % 2.
For complex numbers, the notion between "modulus" and "size" goes through the notion that the modulus is a norm, and norms and sizes are intuitively linked. For the split-complex numbers, the modulus is not a norm, so the link between modulus and size does not exist.
You can think of the modulus operator as giving you a remainder. count % 6 divides 6 out of count as many times as it can and gives you a remainder from 0 to 5 (These are all the possible remainders because you already divided out 6 as many times as you can). The elements of the array are all printed in the for loop, but every time the remainder is 5 (every 6th element), it outputs a newline ...