Security is about defense in depth. Providing physical security as well as a well-designed network, control over the users and processes on the host itself, and regular maintenance can go a long way towards providing good security.
In the most basic sense, a system is secure if it does what it's supposed to do, even if its users attempt to do something they're not supposed to do. It protects the information stored in it from being modified either maliciously or accidentally or read or modified by unauthorized users.
Consider the security of your household. Perhaps you have an alarm system, but does it work if the intruder cuts the system power? Security involves tradeoffs. How much is your data worth? Does it make sense to protect your system with the level of security you might find protecting Fort Knox, or would that cost more than the data itself? Guardian Digital provides an extremely functional e-commerce server, while still retaining all the reliability, configurability, and scalability you have come to expect with the Linux operating system.