DoTA 2's "new engine" is presumably the long-awaited Source 2 engine.
Valve has just announced a rather expansive update to it's popular MOBA: DoTA 2. Dubbed "DoTA 2: Reborn" the long-speculated update will bring a new engine, new UI, and heaps of new features to the game.
"Everything in the dashboard has been completely redesigned utilizing a new UI framework, and the engine that powers the game has been fully replaced," said Valve in a blog post. The new engine is presumably the first iteration of Valve's Source 2 engine, which will be made available free for content developers when it fully releases.
DoTA 2: Reborn will first be made available as a beta, separate from the original game. "Throughout the beta we'll be releasing frequent updates in order to fix bugs and make changes based on the feedback you provide. As you browse around in the new client, you'll notice a few things that aren't fully functional yet. Additionally, our focus has been on the dashboard so you'll find the in-game HUD currently looks the same," explained Valve.
You can check out some of the new features in the beta, including custom games and a fully revamped tutorial, here. More information will be revealed throughout the week.
The beta itself should launch at the end of the week.