If you need to download some video from YouTube or another website that you can just play and watch video without download it, you can use XDM app for do this.
This application with a chrome extension can help you get download link.
For install you tow way
1 : use repositories
sudo add-apt-repository ppa:noobslab/apps
sudo apt-get update
sudo apt-get install xdman-downloader
And second way is download source file and install
Go to thin link and download tar file
cd extracted folder and run this command