What does it means by cloud server?

Hello guys, Please help !!!
this is stape io pricing chart…
Could you please tell me what does it means in cloud server?
Can I setup for 3 website if I buy pro packages?

Google recommends a minimum of 3 servers per container for redundancy. Server-side tagging  |  Google Tag Manager - Server-side  |  Google Developers

That is why we set up 3 cloud servers for containers using a Pro plan and 5 cloud servers for the Business plan.

That means one purchased account is for one website only, right?

yes, correct. One sGTM container for one website.