Your private spend key — often called the "Mneomnic seed" — is literally just a number. Simply knowing this number allows anyone to restore your wallet, see your history, and spend your funds! But what are the chances someone will guess your secret key?
The private key is a 256-bit integer.
In decimal, that's 2256 = 115792089237316195423570985008687907853269984665640564039457584007913129639935
In scientific notation, that's ~1.15 x 1077
In spoken English, that's a little more than one hundred thousand trillion trillion trillion trillion trillion trillion.
To guess someone's secret key, you need to guess the secret number, basically between 1 and this beast.