HQPlayer is an application that allows you to apply upsampling, format conversion and most-uniquely, a wide array of filtering and noise-shaping to your music files.
Roon has built-in options for upsampling and format conversion (e.g. PCM to DSD or DSD to PCM), and for some filtering options, but it is not as flexible nor as powerful as HQPlayer’s options.
It’s worth noting that the higher levels of upsampling, and especially the high-order filtering and noise shaping options HQ Player offers, require significant computing power (either a multi-core setup or solid GPU-compute capacity) and ideally a machine dedicated to the task.
HQPlayer is most interesting if you’re using a DAC that is either of NOS configuration (non-oversampling) or that can be driven in a manner that bypasses any internal oversampling/filtering. Some people report major improvements in the sound they get, especially with native DSD DACs, though my experiences have been more along the lines of “sounds different” than necessarily “better”.