What is it?

The debug button was added to help uses debug their code with different stages.

Scratch Wiki wrote:

Debugging Help is a dialogue in the Scratch Program that gives help on debugging scripts. It was added as part of a major Scratch 3.0 update on the 16th of December, 2024.

The Debugging Help dialogue can be opened by clicking on “Debug” in the Header.

When clicked, the debug button opens a dialogue with 13 different sections including different tips to help the user understand the issue within their project. It goes through different ways the user may have made a mistake in their code, and gives instructions on different methods to try to fix their project, like timing, sound checkpoints, and adding comments to the code.

It basically teaches strategies for fixing bugs in your code - I find it really helpful as someone who gets frustrated easily

Roblox888i_phone wrote:

What is it?

The Debug button at every project editor is a guide for how to fix code glitches.

how dos it work?

Lavit8 wrote:

how dos it work?

I think the replies above explain it fairly well.
If you are looking for further information, here is the Scratch wiki topic on the debug button: Debugging Help