Reconciliation is a term in React that refers to the process by which React updates the DOM with the results of a render. Here's a more in-depth explanation:

When a component's state or props change, React needs to determine whether an actual DOM update is necessary. It does this by creating a new virtual DOM and comparing it with the old one. This process is called reconciliation.

1. Diffing Algorithm: The first step in the reconciliation process is the diffing algorithm. React iterates over both trees (the old and new virtual DOM) simultaneously, depth-first. When it finds two components of the same type, it keeps the same underlying DOM node and only updates the changed attributes. If they are not of the same type, it removes the old node and inserts the new one.

2. Keys: React uses keys to identify elements. When children have keys, React uses the key to match children in the original tree with children in the subsequent tree. Keys should be stable, predictable, and unique for every set of children.

3. Component Lifecycle: During reconciliation, component lifecycle methods are also invoked. For instance, componentWillUnmount is called on any instances that are being removed, and componentDidUpdate and componentDidMount are called at the time when instances are inserted or when they are updated.

4. Optimization: React strives to minimize the number of mutations when diffing the trees to make the update as efficient as possible. It makes some heuristic assumptions, like two elements of different types will produce different trees, or that an element with a key will be unique among its siblings.

The reconciliation process allows React to minimize its interactions with the actual DOM, which are costly in terms of performance. Instead, most of the computations are done with the virtual DOM, which is much faster. This is what allows React to be so performant and is one of the reasons why it's popular for building user interfaces.

Summary:

Reconciliation is a process in React that allows it to efficiently update the DOM. When a component's state or props change, React needs to determine whether an actual DOM update is necessary. It does this by creating a new virtual DOM and comparing it with the old one. This process is called reconciliation.

In this process, React will traverse both trees of elements (old and new virtual DOM) simultaneously and generate a list of mutations (changes) that will make the old tree look like the new one. These mutations are then applied to the actual DOM.

The reconciliation process allows React to minimize its interactions with the actual DOM, which are costly in terms of performance. Instead, most of the computations are done with the virtual DOM, which is much faster.