Linux,as a an OS, is a tool. It is very much open unlike the other operating systems that you could potentially use. Being open, Linux has the capability to be very secure, not in the sense that their won't ever be attacks that target it, but in the sense that there are more eyes on the project, helping to keep it updated.
As an OS the custimizability is amazing, allowing users to create a system that can take on a multitude of tasks from serving up content, to consuming said content to providing simple machines performing simple tasks.
Linux itself is the kernel providing the guts of the OS, a kernel being the interface of applications and software to the underlying hardware of the machine that it resides on. I personally do not know much about the kernel beyond that, other than it runs on most everything these days.
The Linux ecosystem to me means choice. From the distribution you choose to use, to the use of the machine and how bloated or trimmed you would like it to be, you can make it happen with Linux.