Question

What is happiness if we want to evolve some Pokemon?

Answer 1

Happiness is just a number value for each pokemon. You can increase it by having pokemon in your party, battling with them (if your are in johto) giving them haircuts. It can also go down if you let your pokemon faint a lot. There is a person in every game who tells you about a pokemons happiness level.

Answer 2

Happiness is one of a key part for evolution for certain pokemon. every time your pokemon win a battle, it isn't just gain exp points, but it gain happiness points too. after your pokemon reach it's max happiness point, it will evolve. if you want your pokemon happiness grow faster, here my advice!
-put your pokemon in the first party
-win every battle and don't lose, and
-if you have an item called "soothe bell", give it to your pokemon to hold, it will raise your pokemon happiness faster!
hope this usefull...