Up to this point, those who've shared video on their Twitter profiles have had to paste it in from a competing platform, or from Vine. The problem with Vine, though, is that its clips are limited to 6 seconds. According to Re/code's sources, Twitter's planning to test the waters with regards to allowed video length, but it seems at this point that 20 seconds is the target. See? YouTube has nothing to fear.
When this feature arrives, the official Twitter mobile app will be updated accordingly. Anyone wanting to take advantage of the video feature would then be able to record their video through the app itself, or search for a pre-existing video on their device for uploading.
If this feature seems a little boring, it's probably because it is. The reality is, this feature should have existed long ago; it's quite surprising just how long it's taken to get here, especially with competing services like Facebook and Instagram having had native video sharing features for some time. Better late than never, I suppose.