What type of battery is this?
In the RC world, S means number of cells connected serially. A &quot;1S&quot; cell is a single Lithium-ion cell that equals ~3.7 volts. A 2S battery is two cells connected serially for ~7.4 volts.
A DC jack is more ambiguous. It could be a DC barrel jack or any of a variety of connectors commonly used in the RC markets. Just make sure you observe correct polarity.
In your case (now that I checked out the video briefly), if you're wanting to power an Arduino via its DC jack, then you'll want a 2.1mm inner diameter, 5.5mm outer diameter with positive tip style DC barrel jack.
Edit:
Based on comments, here are additional thoughts and information.
First, a bit of advice regarding questions about videos on this site. When you ask a question about parts in a video and just drop a link, it requires anyone who wants to answer to have to painstakingly go through the video. Most do not spend the time doing this which means you should capture screenshots, write detailed descriptions/synopsis, or at least link to specific times in the video.
Next, videos that simply show assembly of something with no narration (just annoying music), no links to time indices, and very little information in the description, are, in my opinion, very lazy. They're as bad as a component without a datasheet. They make it very difficult to find relevant sections and part information. But that's the difference between hobbyist DIY-ers and professional engineers. That said, I believe the video author assumes to some degree that anyone watching will have some familiarity with various battery connectors and just use whatever they have.
With that out of the way, here is a screen cap of the particular battery connector in the video:

There's no information given in the video about this connector, but I believe it may be a JST RCY connector. (JST is the brand, RCY is the series or model.)
You're free to remove the connector from whatever battery you have and use something else. Just be sure to observe proper polarity and use something that is rated for the current and voltage.
The Arduino Uno used in the video has the following pins:

(Source)
The orange and red pins in the POWER section are used to power the Arduino as alternatives to the USB and DC barrel jack. It has an on-board 5V regulator which is meant to drop higher voltages supplied to the barrel jack to 5V. The Vin pin (sometimes labeled 9V) is connected to the input of this regulator, along with the center pin of the barrel connector. This is where you should connect the positive wire from the battery. According to Uno's official documentation, the recommended voltage range is 7-12, and a maximum range of 6-20.
The 5V pin can be used to supply the Arduino with 5V, or can be used to supply something externally with 5V after the regulator (however keep in mind it is extremely low current).