ShouldComponentUpdate

"shouldComponentUpdate" is a lifecycle method in React JS. It determines if the component should be re-rendered or not. It is a performance optimization method that can be used to let React know if the output of the render function of a component needs to be changed or not.

The typical use case is when your component has props that do not change, which means your component does not need to re-render since nothing has changed in its props or state.

The "shouldComponentUpdate" method takes two parameters - nextProps and nextState. These parameters represent the props and the state the component will receive and/or have in the next update.

The method returns a boolean. Returning false will make React skip rendering the component and its children.

Below is an example of how you can use it:

```
class MyComponent extends React.Component {
shouldComponentUpdate(nextProps, nextState) {
if (this.props.myProp === nextProps.myProp && this.state.myState === nextState.myState) {
return false;
}
return true;
}

render() {
// ... render logic
}
}
```

In this example, the MyComponent won't be re-rendered unless its prop "myProp" or its state "myState" changes. This can drastically improve performance especially for large complicated components, because it reduces unnecessary render calls.