It is a feature that allows one interface to be used for 
general class of actions. The specific action is determined 
by the exact nature of the situation. In general 
polymorphism means "one interface, multiple methods", This 
means that it is possible to design a generic interface to 
a group of related activities. This helps reduce complexity 
by allowing the same interface to be used to specify a 
general class of action. It is the compiler's job to select 
the specific action (that is, method) as it applies to each 
situation

The above answer is well explained. In technical terms it
can be explained as same function name with different
signature. Signature means only interms of arguments, and
not for return types. The idea is very simple and is a
feature which make the end user comfortable, and as  Swetcha
said, it is finally the compiler will decide, which function
to call when, according to the type of data passed to the
function. 
eg. int add(int, int)
    double add(double, double)
    char add(char, char)

but the following is not possible
int add(int, int)
char add(int, int)