To store data your server is currently working on, for example active players or currently loaded chunks, it requires something called RAM, also commonly referred to as memory.
What is RAM?
Unlike storage, which describes the space used to store files and folders on a disk, memory is not permanent. When your server is done working with some data, it will store it back to the disk and free up the memory to be used for other tasks. When your server is offline, it doesn't need any RAM, as all important data is stored as files on the disk and the server doesn't have to compute anything.
Java, the programming language that Minecraft Java Edition is built in, often keeps this data around as long as the memory isn't needed otherwise because freeing up this memory prevents it from doing other tasks. This means that your server can usually free a lot of memory that's no longer required when there is little free memory left. It might also show most of your RAM as used, even if it has enough space to work with.
How much RAM do I get on Aternos?
The amount of memory assigned to your server depends on which software and version you use. A Vanilla 1.20.1 Server for example receives 2400 MB of RAM. Different versions or software installations require different amounts of resources, so we try to provide a good experience on every software by individually assigning them different amounts of memory.
You can see how much RAM is currently assigned to your server in the server section, while your server is online:
Generally, more modern Minecraft versions and modded servers require more memory. Changing your server to whatever software has the most memory is therefore also not helpful to your performance. If you want the best performance on your server, use Paper or Purpur, even with less memory than modded servers they provide superior performance overall.
How do I get more RAM?
As a free service, we only have limited resources and want to share these fairly among all users. It is therefore not possible to get more RAM on your Aternos server. If you require more RAM, then you might need to check out a paid host like exaroton.