An "hero project" is a project/spec change that you personally think will solve a group of problems in your project. It could be a different architecture, a new framework or even a new language.
Hero projects happen mostly when a single developer wants to prove something without the support of the company or even the time they are in.
On those projects, developers will spend their free time to write a proof-of-concept, just to prove a point.
And, sometimes, it just proves that they are were wrong.
(Although that last point sounds a bit sad, if you have to do an hero project, you'll still learn something new and, maybe, even add a new bullet point to your CV.)
Just to be clear: Sometimes an hero project will fail because the answer is obvious. Don't let that make you feel down.