What do you mean by underflow and overflow of data?

Underflow and overflow of data:

When the value of the variable is either too long or too small for the data type to hold, the problem of data overflow or underflow happens. The largest value that a variable can hold depends on the machine. As floating point values are rounded off to the number of important digits allowed, an overflow results in the largest possible real value while an underflow results in zero. C does not give any warning or indication of integer overflow, it easily give erroneous result.