It's about a data type in the code or data.
The thing is that a signed int (32 bit signed integer) takes values between -2,147,483,648 and 2,147,483,647,
while unsigned int goes between 0 and 4,294,967,295
Clearly Satoshi meant that in case an error is found in his code, that has to be fixed before a big number is reached (bigger than 2,147,483,647), because in case of signed int that will mean an
overflow to negative numbers.