A Client Side Script is a programming language that performs its tasks entirely on the users machine, and doesn’t need the use of the server to function. As long as the website you’re accessing has been loaded, you’re still able to browse this page if the internet cuts out, you won't be able to load a new page as this requires the use of server interaction which requires the internet.
Magecart is a group of malicious hackers who target eCommerce sites and steal customer payment card data.
To read more about Magecart malware here, click the link here.
You can read their their Security Alert here.
You as a web developer or eCommerce manager/owner, will have to balance the amount of interactive features you want on your website and the risk of getting breached.
Nowadays, security is just as fundamental as engagement to the success of a website.
For monitoring and alerting check out FGX-Web.