It’s just what it sounds like: the driver is Free software. This is in contrast to the situation with Nvidia where there’s a Free software driver that doesn’t perform well and a proprietary driver from Nvidia that performs better, but is kind of a pain in the ass for users and distro maintainers to maintain. You’re reliant on Nvidia for support so you’re forced to use certain versions of kernels and libraries (I think). The AMD driver is free, open source, performs well and is more flexible.