Simply: how you get your phone to play videos it’s not supposed to know how to play. And: That’s about the longest headline in a while.
It’s like this: a default video player in a HTC phone can play set types of formats of video. The most common formats are supported, and some are not supported by the default player due to various reasons too tedious to get into there.
So, how do I play things my phone can’t play with stock player?
The easy fix (and the correct fix). Is to install a player that can handle it. And there’s two commonly used players that tends to know how to do play anything: VLC and Mx player. I prefer VLC myself.
And, that’s pretty much the fix for it. Download a player. Easy fix as well.