What is doInit in lightning component?

This event is automatically fired when an app or component is initialized, prior to rendering.

Component-

<aura:component>

<aura:handler name=”init” value=”{!this}” action=”{!c.doInit}”/>

</aura:component>

jsController-

({
doInit: function(cmp) {
// Set the attribute value.
// You could also fire an event here instead.
alert(‘test’);
}
})