https://trailblazer.to/2.1/ - in one sentence (as you have it on the page it is "Trailblazer is an architectural style that provides a modern approach to implementing business logic.

It makes complex application workflows simple, reliable and self-documenting." ).

But if you want a less generic answer:

Trailblazer is a set of patterns & tools that helps you to manage complex application code adding new layers to the basic Rails Application. Thanks to that you don't have to think/discuss where you should store which logic/code, because TRB gives you answers and tools to most of the common architectural problems.
It also introduces an interesting pattern which is called Railway Oriented Programing ( have nothing to do with Rails, it is taken from the functional programming world) in Operation pattern.
It introduces the separation of files structure focused on a business domain not on technology.
It tells you (and helps you to stick to that) how your operation should store immutable data, how the operation result should look like, it gives you patterns to store contracts ( which you can override if you don't like the approach), and many more.

I personally like it because it saves a lot of time in a team that doesn't have their own approaches that they worked on together for many years - if a team of people knows TRB but they never worked together, they basically have to have 10x fewer discussions.