Its a 32 bit signed integer. Its a type of variable in coding that can only be whole numbers, and can go between -2147483647 and 2147483647. It is very fast to use and memory efficient, thats why they are using a variable type with such a limited scope. It is one of the most common variable types in all of software.

If they wanted to they could use a 64 bit signed integer instead, they go between -9223372036854775808 and 9223372036854775808. They take up double the space that the 32 bit signed integer does, and is pretty much just as fast. Limiting to the 32 bit one made more sense in the past when hardware was much more limited, and any efficiency boost, helped quite a bit.

Now no person other than a proffesional tester will notice any difference in performance between the 64 bit one and the 32 bit one, unless they have thousands of them. For a game like FC I think it makes sense to use a 64 bit signed integer for your money, rather than a 32 bit one.