Just like in case of the function arguments, template parameters can have their default values. All template parameters with a default value have to be declared at the end of the template parameter list. The basic idea is that the template parameters with default value can be omitted while template instantiation.
Simple example of default template parameter value usage:
template <class T, size_t N = 10>
struct my_array {
T arr[N];
};
int main() {
/* Default parameter is ignored, N = 5 */
my_array<int, 5> a;
/* Print the length of a.arr: 5 */
std::cout << sizeof(a.arr) / sizeof(int) << std::endl;
/* Last parameter is omitted, N = 10 */
my_array<int> b;
/* Print the length of a.arr: 10 */
std::cout << sizeof(b.arr) / sizeof(int) << std::endl;
}