CPU Core is the percentage of a processor core that your CloudServer can use. If you plan on hosting busier web sites, or a great number of websites, then you'll need to increase the amount of CPU.
RAM stands for random access memory, and is like your server's work area. The bigger it is, the more things it can work on at the same time. Again if you expect your site to be busy or you plan on hosting many sites, you'll want to add more RAM.
Space is the amount of storage you have for your cloud server. Some of it will be taken by the operating system, the rest will be available for you to store data. The more data you need to store, the more space you'll need.
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