DOM
In plain terms
A live, tree-shaped representation of a web page that code can read and change.
Going deeper
The Document Object Model turns HTML into objects browsers and JavaScript can manipulate, so scripts can add, remove, or update elements on the fly. Every dynamic page edit goes through the DOM.
Under the hood
Direct DOM manipulation is slow at scale, which is why frameworks use a virtual DOM or fine-grained reactivity to batch and minimize real updates.
Examples
document.getElementById, addEventListener, React's reconciliation, jQuery selectors.
Think of it like...
Like a building's live blueprint that updates in real time: change a room on the blueprint and the actual building rearranges itself instantly.