As far as reading this chart goes this shows your traffic over the last 28 days. You can change the amount of time in the upper right corner (from "last 28 days" to a number of other lengths of times).
The actual chart shows watch time (in minutes) per day, and then breaks it out (by color) by various traffic sources. You can see the numerical representation of this in the table below. The purple means that a lot/most of your traffic comes from people watching your videos from a channel (because see in the table how "Youtube Channels" has a purple line to the side of it?). (If you click on the "YouTube Channels" link in the table, it'll tell you which channels...I'm guessing most if not all of it will be your channel.).
58% of your watch time (as in minutes watched) and 64% of your views (as in every time someone clicks on a video, regardless of how long the video is in minutes) comes from your channel. The "Average Percentage Viewed" shows how much on average someone watching videos via your channel will stay for...here, people are only watching (on average) 25% of your videos before leaving.
In contrast, 20% of your views and watch time comes from "External." (If you click on that, you can see a vague breakout of what external sources there are) Even though they make up less of the total watch time or less of the total view count, they watch 41% of your videos. So, something is happening where people who aren't really interested in your videos are finding your channel, only watching 1/4 of what they find, and leaving. On the other hand, people who find your video through external sources, suggested videos, or Browse Features tend to like your videos more and they stay for a bit longer.
As
@xingcat has explained, what you can learn about your statistics are a few things: 1) only around 1% of your viewers are finding your video through Youtube search and 2) people who find your videos are not likely to stay very long. Based on your highest sources, I would guess that much of your traffic comes from people clicking links directly to your channel, or if there are links to specific videos on external websites. However, this would be a very time intensive promotion process going in the future -- getting your videos set up where they can be easily found through search is usually a better bet in the long run, because that's a passive stream of traffic.
Proper titles, tags, and descriptions will help your videos show up in the search results early on, but Youtube looks at audience retention and time watched statistics to determine how videos should rank as time progresses...so if Youtube sees that most people don't stay very long on your videos, they won't rank them highly in search, so fewer people will find them, and so you'll have lower views.
thanks for those who will be helping me understand.. 
