JRemote2 is a Media Center remote app like Gizmo and MO 4Media, which like them requires JRiver Media Center running on another PC/Mac to connect to (and you can either control that Media Center running or stream music from that running Media Center to the Android phone). JRemote2 is unable to playback media files that are locally on the Android device itself (e.g. music files on a SD card).
JRiver for Android is a standalone media player for Android devices. It can import and play files located locally on the Android device and it's capable of loading libraries from other Media Center instances running on PCs/Macs. JRemote2 and MO 4Media are the two best remote apps for JRiver Media Center right now. I have to use JRemote2 over MO 4Media (and Gizmo) when playing back "hi-res" files (e.g. 24/96 and higher) because for some reason on my phone trying to play them audio skips and it simply isn't able to play them. JRemote2 on the other hand can, and I suspect it's because it's using ExoPlayer 2 for playback.
I use both JRemote2 and MO 4Media to stream FLAC files (gaplessly!) from my JRiver Media Center running 24/7 on a Raspberry Pi (which is linked to my NAS) to my Android phones. It works extremely well - I can even enjoy Pink Floyd albums without any annoying gaps between songs.