Audio pass-though means that Vimu Player (or Android system) does not really decode audio stream by itself, but rather passes it over HDMI or optical/cable connection to a decoder device.
Decoder device can be TV, AV receiver or sound bar. The main reason for passing sound is to get surround sound from your 5.1/7.1 system or sound bar.
These formats could theoretically be passed over: Dolby Digital, Dolby Digital Plus, Dolby True-HD (incl. Atmos), DTS, DTS-HD. We say theoretically, because possibility greatly depends on an Android device.
Basically, AC3 pass should work with almost any Set-top Box and TV set. This includes all Android TV certified devices, Fire TVs and most of the Android-based TV Boxes.
Unfortunately, there is no strict way for pass-through in Android. That directed us to implement three modes for passing sound: Auto, RAW over PCM and IEC. Most of the devices support only one or two of them. But not all.
Most Android TV certified units and all Fire TVs can pass-through AC3 and EAC3 with this mode. Less devices support DTS. Some (like Nvidia Shield) also support DTS-HD. You should try this mode first before switching to other modes.
IEC here actually means IEC61937. This is the most recent improvement in Android system (started from Android N). This in theory allows passing any format over HDMI, In real life it not widely supported yet.
RAW over PCM
Before Android L, there was no support for pass-through at all. So, device makers used some tweaks. The most obvious way was to output compressed sound as it was a decoded PCM. Most receivers can detect this by looking for special tags in the bitstream. But sometimes, it may lead to a white noise. So, this mode should be used with care. In some cases it is also important to make volume 100% in Android system to avoid breaking bitstream.