What is the BigNum and how can we work with very big numbers,like “find the sum of the even-valued terms of fibonachi sequence that the value doesn’t exceed 4 million” that we cannot put it in long long int?
"BigNum" is a concept that describes the data structure that represents an arbitrary-precision number. This data structure is usually implemented as an array of digits of variable length. You can read about it here Arbitrary-precision arithmetics.
The only limit for such number representation is the amount of memory a computer can hold, so, very big numbers can be handled using this "BigNum". 4GB -> maximum number possible is (approximately and depends on the implementation) 2^(4 * 1024 * 1024 * 1024).