Am sure you are aware that once someone send or share a video to you on whatsapp, you will need to download it first before you can be able to watch the said video but WhatsApp is currently working on new update that will allow users to stream/watch video without downloading it first.
This feature is already available on WhatsApp beta version 2.16.365 And WhatsApp Mod, so instead of a download button, you see a play button. When you tap the button, the video will start buffering, and you will be able to watch the video while it is being downloaded on your phone. This video buffering feature is the same thing you see when watching videos online.
As at the time of writing this, WhatsApp video streaming is only available on the Beta version which you can download from Playstore.