it is a string "115792089237316195423570985008687907853269984665640564039457584007908834671663" or something else..?
It's the value of the moduli. It is the biggest number in the curve secp256k1 that bitcoin uses.
=2^256-2^32-2^9-2^8-2^7-2^6-2^4-1
If your generated private key is bigger than that, then you have to subtract that number from your generated key as many times as necessary, until it becomes smaller than that number.
The security of bitcoin depends on big numbers. That is the reason why the moduli has to be so big.
https://en.bitcoin.it/wiki/Secp256k1