Dismissing is one way to get rid of unnecessary familiars you acquired either throug pacts or by capturing them in a Zone or a Special Dungeons Event. In exchange for them you get the amount of Gold thats written as 'Worth' on the familiar card.
Dismiss can be reached by Home > More > Dismiss.
|“||Dismissing a familiar annuls its pact in exchange for a sum of gold. If you find yourself with too many of a certain familiar, the gold may be of more use.||”|
|— In-game Help|
Just select up to 20 familiars you no longer need in the main screen and proceed with the 'OK' button next to the total worth of your disposals.
You can sort your familiars by stack, level, skill level and rarity to prevent that you don't accidentally release one of your top ones.
Once familiars are dismissed the process can't be undone. So be sure to double check wich familiars you selected in order to prevent the loss of a powerful familiar.