Streaming is real-time, and it's more efficient than downloading media files. If a video file is downloaded, a copy of the entire file is saved onto a device's hard drive, and the video cannot play until the entire file finishes downloading. If it's streamed instead, the browser plays the video without actually copying and saving it. The video loads a little bit at a time instead of the entire file loading at once, and the information that the browser loads is not saved locally.
Think of the difference between a lake and a stream: Both contain water, and a stream may contain just as much water as a lake; the difference is that with a stream, the water is not all in the same place at the same time. A downloaded video file is more like a lake, in that it takes up a lot of hard drive space (and it takes a long time to move a lake). Streaming video is more like a stream or a river, in that the video's data is continuously, rapidly flowing to the user's browser.