zlib license questions

[Cube_]

Exactly what does this mean?

..Permission is granted to anyone to use this software for any purpose,
including commercial applications, and to alter it and redistribute it
freely, subject to the following restrictions...

For example, if someone were to make a heavily modified [or even lightly modified] version of irrlicht and wanted to release that - would they have to release under zlib or could they release under another license like GPLv3?
What does "redistribute it freely" even mean? That's the most vague thing I've ever seen in a license.

[CuteAlien]

You can't remove the license. But you can put an additional license on it (like gpl3).

[Cube_]

Oh, ok - I thought so but I wasn't sure, and it's always a good idea to get advice on such questions anyway.

[CuteAlien]

I'm not a lawyer either ;-)

[Cube_]

That may be true, but you're likely more knowledgeable on the topic than I am - given that you actively contribute to a zlib project.