Full name of RMVB is a Real Media Variable Bitrate. This is one of the newer formats from Real for publishing video files, which is quickly becoming a popular format because of smaller file sizes and lower bitrates with very good quality.
You can playback the file by downloading and installing the latest version of RealPlayer, a free edition this software available here.
After downloading this software, install this and associate the RMVB file type with it because it isn’t recognized by default.
You can also use real player Alternative for playing RMVB file. You can download here.