This refers to a technical concept, not as a generic "container system" or a "interactions system"
A system is a classthat manages something, it should work on its own, the only exception is when it needs information about other system, but it is preferable that you do that by using eventsor event buses or network messages.
A system can be networked or not, for that you can inherit your class from System or NetworkSystem.
Here's a snipped of now you can declare a system that creates explosion somewhere.
It is also important that you know all the systems work with the class
Always add the postfix "System"in the name of the class for your systems.
public sealed class ExplosionSystem : NetworkSystem
{
public void CreateExplosionAt(Vector3 position, float size)
{
// boom.
}
}