What's the difference between them? Let's take a look:

Hard navigation

Browsers perform a hard navigation when navigating between pages. This is the traditional way to navigate between pages. The whole page is re-rendered on the client and the React state is not preserved.

Soft navigation

In soft navigation only the route segments that changed are re-rendered. This preserves the React state during navigation. This is what happens with partial pre-rendering in Next.js app router.