How to use Explicit Constructors???

Question

What is the use of declaring "Explicit Constructors",why not use normal constructors,???

Answer 1

https://www.youtube.com/watch?v=Rr1NX1lH3oE&t=411s
https://stackoverflow.com/questions/12437241/c-always-use-explicit-constructor
http://www.cplusplus.com/forum/general/168292/